Well, I actually got into bluegrass the middle of last year. It's a hoot. I can tell you a few things though...
Firstly, though it's not common practice, more and more bass players are using electrics in bluegrass. So don't get too hung up on having an upright.
As for the focus, it's very simple but hard to apply. Typically, you play half notes going from I(1) to V(5) that's the umpah - umpah that you hear in from the Tuba in polka.
To use terms that are easy to understand you(the bass) and the mandolin tend to emulate what a drummer would do. You hit the down beats simulating the kick drum of a drum kit and the mandolin hits the up beats acting as the accent note of the snare or high hat.
To function as a bass player in bluegrass that's all you need to know. Knowing more, like the walking basslines, is a plus though. I'm self taught, so I cna't really help you much on where to get such materials... I would maybe recomend checking out this website though
http://www.fqms.com . My bandmates pretty much swear by it and they have a heavy focus on bluegrass and country.
As for buying a bass, check that website above. Also go to festivals! You can meet people that make them, some are very affordable compaired to buying from a store. You'll also always find people selling instruments. I have seen some basses go pretty cheap at festivals before.
If you have any questions at all, feel free to contact me. I love to help
