I've got a little Mitsubishi MT180 which stands for 18 hp and many riding mowers are also 18hp but there's a world of difference as you say. I also have a couple of old Sears Suburban garden tractors. I don't call them riding mowers because they have a high and low range for the transmission and they will literally pull a full size truck on level ground and mine are only 12hp though later, they went on to make them in 14, 16 and 18hp. They have a three point hitch, Cat 0 and it's a lever, not hydraulic lift. Still, I did a ton of work with it, including making my driveway with the grader blade. The "PTO" they have is merely a couple of extra pulleys on the engine that runs things like a mower. Those old garden tractors from the 50s, 60s, 70s and maybe a few from the 80s, are heavy duty and can do a lot, plus you can get one a lot cheaper than a bonafide tractor. On one of two I have, I put dual rear wheels with chains on them and plowed the snow off of the driveway, pushed large piles of brush. People have put small hydraulic pumps on them and built a front end loader. The Sears' weigh 750 lbs and has a heavy 3/16" plate frame unlike a riding mower which is thin stamped out sheet metal.
Of course the Mitsubishi has rear tires that are a lot bigger and it has the hydraulic 3 point hitch, Cat 1 (Category 1) and the real PTO. It's got a little 3 cylinder diesel that as you say, sips fuel. I can mow all day on 2 gallons of fuel or if I'm just driving it around toting stuff, 3-4 days. The only implements I have are a rear mount finish mower and a dirt scoop aka poor man's loader. I use the tractor for carrying stuff as much as anything else. Usually with the dirt scoop but on a lot of items, like my wood stoves, I can just back up with the lift arms under the stove, run a chain from where the top link mounts on the tractor to the top of the stove, lift and drive away. I also use it for moving trailers around the yard. With the dirt scoop, I can throw my chain saw in and head back to the woods, cut down a tree, get the scoop under one end, lift it and set something under it to prop it up to cut up
firewood sized pieces, fill the dirt scoop with wood and bring it right to my back door if I want. I can drag huge logs, 16" dia x 24 foot long. I'm building a stone foundation for my shop(future barn) and use the dirt scoop to haul big rocks, like 300 lbs big. I've moved an engine on an engine stand by running a pipe through the lift arms and the sq tubing at the bottom of the stand and running a chain to the top, like with the wood stoves, picked it up and moved it like it was nothing. I do the same with my engine hoist. Bought a fridge from my neighbor a couple of miles down the road. Went down with the tractor, tilted the fridge back and got the lift arms under it, used a strap up high instead of the chain and drove home.
The next thing I want is a set of forks.
Pallets are free and a small pallet like 3' x 3' would be a perfect fit and can have sides put on it for a 3x3x3 tote. My tractor doesn't have the mid PTO and while there is a place to tap into the hydraulic system, I haven't done that yet. Some day, I'd love to get a hold of an MT180D, the D means 4x4. The other thing I use the tractor for a lot is toting
water. We don't have a well yet so I mounted a 225 gal tank, a generator and sump pump on a trailer and use it to go down the road to a spring fed creek. There's quite a hill to climb on the way back and it's a gravel road. The tractor weighs 1100 lbs and the water, gennie, trailer weigh about 2000 lbs. Took a couple of tests to get the tongue weight right to be able to get up the hill without tires spinning.
So yes, if you can get a deal on a sub-compact or compact tractor, they're handy to have and a good set of attachments is worth as much as the tractor, especially since the smaller attachments are not as plentiful as the standard sized.
If you can't swing a small tractor and happen to run across a deal on an old garden tractor, those will do a ton of work too. There's even some with a PTO that comes out the back that will run a tiller. Snow blowers and snow plows are common attachments for them also. The Ford LGT series had diesel engines and rear PTO and I think some of the old Power King brand had PTO as well. Again, implements/attachments are the key.
I took a bit of a chance in buying my tractor as it hadn't been run in 4 years so I didn't hear it run before I bought it. The seller swore it ran when parked so I bought it for $900, no attachments. Got it home and spent a day getting it to run and spent another half a day getting the lift arms freed up. Not too long after that I was lucky enough to find the 4 foot finish mower. A 5 foot mower or brush hog would be too big for 18 hp and the 4 foot mower was blue and white, just like the MItsubishi so they look like they belong with each other.
Since I know it can pull a ton(literally), I plan on building a long narrow trailer for hauling firewood up from the back 40. The axle will be about 2/3 of the way back on the trailer to give me plenty of tongue weight which gives traction.
I even made a
chicken plucker for it. I borrowed the gear box from the finish mower and ran it backwards so it was geared down instead of up. Mounted the bottom plate with plucker fingers and put a plastic barrel with more finger on it and she plucked away at a slow idle.
For firewood, I want to build the long narrow trailer I mentioned and would also like to put a splitter on it. Then I could go cut a tree down, cut it up, split and toss in the trailer. Bring it up and stack it. Right now I'm cutting down, cutting up, hauling as much as I can in the little dirt scoop, which is about the same as a wheel barrow, and throwing it in a pile until I have all I need and then borrowing a splitter. Then split it and move it again.
I've got some curved pipe from a
hay shelter and plan on using it to make a movable high tunnel. I'll be able to move the tunnel with the tractor and also prep the beds and lay plastic mulch with it.
There's quite a few brand names. Kubota of course, Mitsubishi (formerly Satoh), various brands with a Yanmar diesel which are good. One I would stay away from is the Cub Cadet Low Boy. They PTO on them runs backwards so you have to use an adapter for all generic implements and most of them came with no low range and won't go slow enough for plowing.
For garden tractors with a PTO which is an actual spinning shaft out the back that will run a tiller, there's Power King, Bolens, Ford LGT series, Simplicity and others.
The scary thing about buying one of these old tractors or garden tractors is lack of replacement parts. There's one company that has some NOS(New Old Stock) parts for my Mitsubishi but if they don't have it, I'm screwed. I can buy a starter, alternator(I think) and a waterpump off of ebay but that's about it. No internal engine parts unless that one company has it. Same with hydraulic parts, brake shoes, clutch etc etc. Tires? That size isn't made any more and I have dry rotted turf tires, probably original to the tractor which is a 1981. I would have to get some sort of newer rim and maybe cut both sets of rims up and weld my centers into the outer rim from the newer rims just to be able to buy new tires. The only reason I can get a waterpump, starter etc is that they put that same 3 cyl diesel engine in a lot of fork lifts.
There's a company that sells kit to repower a lot of the older garden tractors with newer, more fuel efficient engines.
The old Sears Suburban garden tractors I have, I can pretty much get anything for, used, because they made a lot of them and people part them out on ebay. If you want a good carburetor, expect to pay a few hundred for one. I've seen them go for $500.
As the OP mentioned, sub-compact tractors are very manoeuvrable and will fit a lot more places. Everyone loves the old Ford 8N and 9N and they're a common sized tractor for general small farm use but I wouldn't be able to drive everywhere on my property without cutting a path with that size tractor. My Mitsubishi is 43 inches wide.
One note on the really old stuff like the 8N. The lift arms lift but won't hold mid way. It's either all the way up or drop it and chains are used to adjust how far you want to drop it. With my Mitsubishi, I can set the lift arms at any height and it stays there wit the hydraulics. There's other little quirks with the old ones and some are safety related. There's hardly anything between a brush hog and you. With brush hogs having an open front, chunks and rocks can fly out and hit you.
There are some things a sub-compact will have that a garden tractor won't like independent rear brakes and of course the lift arms and standard PTO shaft size and the ability to lock the axle which makes both rear wheels drive wheels instead of just one aka makes it a posi.
When I was looking for a tractor, I also searched for a Ford LGT 125, 145, 165 (12hp, 14hp, 16hp) as I think those are the most able of garden tractors. Front End Loader, Backhoe and the Tiller were three of the factory implements made for them. I'd still get a garden tractor if it had a loader because those aren't easy to put on and take off and they get in the way if you leave them on. Neither a garden tractor or a sub-compact is going to just dig a hole but they'll scoop up gravel and turn
compost plus you can lift and haul stuff with them.
tractorbynet and yesterdaystractors are good resources for info.