We've got a small herd that includes several mini Nubians. They are wonderful little goats. My only complaint is that, since I have large hands, I find them harder to milk than a full sized goat. Their teat size will vary quite a bit from goat to goat. Just something to keep in mind.
Ask the owner if they have been tested for CAE and CL. Those are two diseases you DO NOT want to start out with. If they can't offer proof that they have been tested, I wouldn't buy, but that's just me. I would also ask if they have been tested for G6S. That's a genetic disorder that is specific to Nubians. The research and testing on that one are fairly new, so they may not have even heard of it. Worth doing your own research on, though.
If the doe is in milk now, ask to taste her milk before you buy. Chances are, it will be wonderful, but it's best to be sure.
Remember that goats are herd animals. You need to have more than one. The other side of that, of course, is that you want milk, you'll be breeding every year, and a single breeding season can double or triple the size of your herd. Think now about what you will do with the babies. You don't want to overload your land with too many goats.
Fencing is a big concern. Some goats can be escape artists, so it's best not to let them get out even once. If they succeed, they'll keep trying.
Watch out for worms! Parasites can kill a goat in no time flat. Pull their lower eyelids down, and make sure the inside is a nice healthy pink, and not pale and anemic. Molly's Herbals makes an herbal wormer that works well, and I believe Fir Meadows does too. But if you get a bad infestation, you may need conventional medicines to treat.
Before you get goats, find a good goat vet. Depending on where you live, they can be hard to find.
http://www.thegoatspot.net has a forum with many knowledgeable and helpful goat people who are always willing to answer questions.
Hope that helps some.