Not necessarily under armor, but sportswear/active wear work fine for me. The seams just bother me, so it helps, plus I can wash it easier than the suit. You can get a set for 5$ at Walmart
If you don't know what kind of brush to get, a lot of suit makers/suiters use a slicker brush. Just make sure you brush it with with the end that curves/bends over going along it, not against it. Most of them you'd push the brush down, with the top part going with the fur, not handle-down. Mostly because its made for de-shedding dogs and you don't want that for a fursuit
Like Ventus said, a fursuit med kit is wonderful
And if you can afford it, a camel-bac or something that can hold water that has a long straw. super nice to have
I'd also, from personal preference recommend using shoes in your suit that are slip ons and fleece lined. I know crocs are really nice in that regard, and most makers can either get that shoe, of you can send it to them would make the feet around that. I made my bat suit with my old laced shoes and its a pain to get on and off.
That's exciting though!!!