Need advice for rebuild project

rgnjc

Hi all, first time poster here. I've owned my Trek 830 Antelope for 10 years now and I've always loved this bike, and I can't seem to find myself interested in buying a new one! I've decided to rebuild my entire bike recently so I took it apart and I've stripped all the paint and any minor rust spots off. My friend is going to give me a professional paint job on it. However, I need some advice on the bottom bracket.

I don't want to go all out, I'm a grad student living meagerly. I want to buy new parts, but not top of the line stuff. I'll take any decent stuff I can find off ebay as long as it looks nice and new (anything is better than what I had). First, I want get my bottom bracket off for painting, but I may ask the local bike shop to do that for me. Second, I was debating whether to replace the bottom bracket, since it sounds "gritty". I cleaned it with solvent though and it sounds better, but heck I went this far I might as well put a new one in while I'm at it! What do I need to know about bottom brackets before I go buy a new one on ebay? I figured I would go with square tapered since that has the cheapest crank sets available. However, I don't understand bracket sizes. Can somebody explain? Thanks!