Besides going to matches, you might want to start thinking about what rifle fits your fancy.
I've gone through this myself in the past couple of years. I've been a single shot collector for a long time. After getting interested in BPCR, I looked at & shot several different traditional single shots, and eventually put a .45-90 Shiloh on order - but also found an original High wall Winchester, also in .45-90.
It might be a good question for those who actually have killed buffalo - what is the minimum suggested BPCR caliber, assuming good bullets and shot placement.