You'll learn pretty quickly with that 50/90, it may not have been everyones choice as a first round to learn BP loading with, but it for sure will teach you.
Brass will cost more than you suspect, so it will be worth it to use techniques that prolong it's life. In general, sizing is rather minimal with BP loading, so most of the war comes from crimps....work towards a minimalist attitude with crimping and sizing.
Bullest will be a bit harder find as commercail cast...and if that is an old Roller, would be best to slug the bore...in fact, slug it three times: once at the muzzle and once just ahead of the chamber and compare two. the third slug is to be driven completley through the bore, trying to tell if there are any tight spots or loose spots along the way. Given a choice, I'd prefer a bullet aht matches (or .001" LESS) the measurment at the BREECH end of the barrel. the logic being that BP is going to bump up a bullet to that size no matter what diameter is starts out life...may as well have it close to the size it will end up to minimalize distortion.
When you look at cast bullets, would be better served with designes with lots of lube (not only the number of grooves, but the depth/width of the grooves). Twist will have a bearing on bullet weight, and the twist rate used on the 50/90's is variable. Even at 50cal. for hunting I'd prefer a bullet with a flat point rahter than the sharper points of target bullets...if you want to shoot paper or steel critters, would prefer the pointed versions for long range.