Hi Kate. I'll preface what I say by admitting that I've never owned a cow, let alone a Highlander, but I've read about them, so I'm an internet expert. I have, though, milked. We've got a quota system here, so most milking is done with Holsteins, but I've also milked a number of other breeds, but not a Highlander. I'm also going to assume that you've never milked before, no shame in that, but my apologies if I'm wrong.
I've found that Holsteins are great for producing lots of lower fat milk when fed to the max and pampered to the gills. Farms around here aren't as huge as in the States, but herds tend to start around 3-400 head and some are milking almost 700 when, 30 years ago when I was a kid, 50 milking was a pretty standard herd. I've had limited
experience with the new Holsteins, but they're much larger and, I've found, not as easy handling as they used to be. I've found other breeds to be nicer handling, but there are always asshole cows in any breed, so you mostly need to make sure you don't get one of those. The last guy I milked for had a pregnant heifer that rammed the other cows. She's already caused one cow to miscarry yet they wanted to keep her until she calved. That's the cow you don't want.
I'd say that your best bet would be to find someone who's willing to
sell a Highlander that's already being milked, or has been milked and is now dry and pregnant. You'll want to buy a girl that is calm, an easy milker and calver, is gentle and can train you. If you've milked before, you can disregard this, but it's so much nicer to handle nice cows, so make disposition a top trait when choosing. If you can find a milking or pregnant cow (a pregnant heifer has never given birth, so hasn't been milked before), I'd spend serious money for a good animal, like a cow on the edge of the top 20-25% of the herd for good genetics. This is a very tough get, especially if Highlanders are rare.
The second suggestion I'd have is to be open to another small breed, though again you can find smaller animals of any breed. The smaller cows have lower nutritional needs, on average, so you can keep them with less
feed. They may not produce as much, but the return may be better than with a high producer needing supplements. I have read a lot about the Highlanders and, here in Canada, we have a pretty good gene pool of them, but they're still a specialty breed and a
dairy Highlander would be rare indeed. I have read a lot of good things about them, but I've heard from a couple of farmers that they were disappointed with the carcass quality. I do not think that either had had success grass finishing with any other breed, so I don't just accept that it's an issue, but they don't develop as much body fat as other
cattle, due to the hair coat, which is a downside for
beef, for sure. That would be a definite trade off for you in raising it for beef, but I get falling in love with a breed. If you're open to other breeds, I'd look for an experienced cow as described above in a small breed. Devons give nice milk, and the breed will have a certain milk profile, but there will be differences in animals and I think that diet has a large influence on the milk quality. Around here in the past, farmers would keep a dairy breed and breed her to a meat breed to get a calf with more muscling ability, plus the hybrid vigor. Personally, I think that the environment and the diet and care of the cow will make a bigger impact on taste and quality than breed, so it may be better to get a better quality cow of a different breed.
If you're stuck on Highlanders, I get it. I will definitely have some at some point, just because I want to see what they're like and I love the looks. That said, my third choice would be to buy a calf, from a good herd, and raise her up. I'd very much recommend that you also buy a great milker of another small breed to train you and the calf. It's best to have two cows, the cow will show the calf what to do if you keep them together, and you can sell her later.
You may be able to get a good cow cheaply if you look for someone who's selling their milk cow. The important thing is not to buy a problem cow, but there's a solution for those as well. Best of luck with your endevour.