I cleared about that much space by hand over a period of years, a few weeks at a time and it slow going. My tools at the time were only a small chainsaw and a Sthil weed whacker alternately fitted with the string trimmer head and large round saw type brush blade. About half the time was spent between refueling the devices, restringing the line trimmer, sharpening the brush blades. The Sthil “bump style” line trimmer heads are pretty reliable, but the moment you hit a piece of
fence or rock it breaks at the opening requiring restringing. The brush blades I have multiples of so I can swap it out instead of sharpening each time it needs it. With the line trimmers it is important to have a good full harness that makes them easier to balance and less stressful on your back.
One advantage of doing it that manual way is you really are in close and see almost every plant and become intimately familiar with every rock, stump and old piece of metal. The disadvantage is you become intimately familiar with every rock, stump and old piece of metal on your property. A session would go something like this, well a wagon containing all the tools, gas and implements to where you are working. You might start with the line trimmer to get you close into the heavy brush and saplings, running it until the gas runs out. At this point you rake your spoils into a pile, sit down in the folding chair you lugged along and swap out the line trimmer head for brush blade that you sharpened the evening before. After gassing you get in and get some real destruction going with the blade. A good sharp brush blade can cut saplings and vines right at the soil level, avoid what we cal “f sticks” named for the exclamation expressed when you trip over it. The blade can handle sapling up to a few inches, but don’t stay sharp long when cutting down small trees. Vines and such succumb more readily to its persuasion, so perhaps you work around the larger saplings and save them for the chain saw.
At this point you have a lot of brush laying around so you have a session of brush dragging to pile it up where it will live and get shredded/munched and/or burned. I’m a fan of piling and leaving. If there are materials like poison sumac or poison ivy there are additional considerations like assuring they are killed or avoiding toxic smoke or burned. Laying down in an out of the way spot and covering to solarize to death by covering is a preferred method. Having helpers dragging brush (and sharpening blades, restringing the line trimmer, etc.) and be a force multiplier, but great care needs to be taken when anyone is around so you do not injure them. Never had a blade fly off the Sthil, and found it fairly safe for the user, the working end being so far away, but it is easy to swing it around quickly and due to noise and ear protection a person or pet can sneak up within the danger zone w/o you being aware of it. One hazard it flying chips or pebbles that could take an eye out, so safety glasses are a must in addition to ear protection. Chain saw safety is a whole long discussion unto itself.
Chain saw work allows you to get at those larger trees after the ground has been cleared so you can safely work around them. Timber felling is one of the most dangerous professions, even with the right equipment. A small tree can kill or injure just as badly as a large tree. Once had a dead branch conk me on the head pretty good just because I grabbed the tree to balance myself on uneven terrain. Working with a spotter and a phone is important, but adds to the effort unless you have free help available.
You mention the desire to create habitats and doing it manually could allow you to build on what nature already has intended for the land. Willing to bet among all that “poison ivy/oak/summac” there are interesting beneficial plants that could be encouraged. Part of the problem is on an overgrown lot it can be difficult to know what you have until you start clearing it. You seem set on clearing it, so perhaps that isn’t much of a consideration and not much info on the specifics of what trees are there and how old they are (all less than twenty?). I might consider doing a careful survey/cruise of the trees (getting advice if needed) and if going the mulched on skid steer route I would be tempted to have
enough done to clear for the plantings and foundation work and flag areas and trees I find interesting to preserve, even if just along the edges.
As to combining the adjacent lot with yours is an important consideration worthy of careful research. Could be more valuable long term kept as a empty building lot, but yearly tax implications could favor combining long term, but would possibly forestall ever selling separately in the future (other have good advice on this).
Owning a two wheel walk-behind
tractor with a brush mower has meant I do a lot less of the back Breaking work as the tractor handles all the little stuff. Someone mentioned the DR walk behinds. They are less expensive than my BCS, but I have the ability to add all sorts of implements to the very capable brush mower attachment. Once the lot is cleared you will need to keep it mowed to some degree, so a walk behind would be useful.