That's kind of a tall order!
It isn't just the gun, but loading equipment, mould, etc. Probably that absolute cheapest to find loading components for is the 45-70. There are lots of guns out there in 45-70. Anything from originals, modern made, etc. However, watch out for the rules of the game. Some relatively good inexpensive guns may not be legal for matches. If you are more interested in hunting, H & R rifles in 45-70 are good quality and cheap, just not legal under the rules of alot of matches. I purchased a used H & R and loading equipment many, many, years ago in 45-70 for $100.00. To buy new today, you will probably have almost $450.00 in it on the real cheap. From there, quality is relative, and features can easily run a gun, sights, and loading gear to $4,000 and up. When BPCR sillywet shooting started, the rules were intended to avoid the expensive equipment race that ruins the fun of so many other shooting disciplines. It happened anyway. The only group I know of that has avoided the equipment race and nit-picken rules folks is a group that has a simple requirement, they only permit original trapdoors, as issued.
There are volumes of stuff available, web sites with debates over rifling, sights, molds, bullets, etc. Lubes. If you have unlimited funds and illusions of grandeur in the sport, indeed buy everything that anyone reccommends. (The economy can use it) Otherwise, remember the old concept of K.I.S.S.
My first 45-70 mold was the Ideal 457125, a copy of an original gov't bullet. It is heavy, carries it's energy over long distances and is better than some at bucking air resistance. It isn't nearly the best, but it is darn good. It has done quite well on paper targets and silly wets and game.
From there on, there is a learning curve that really can get obsessive/compulsive. Rifles, actions, barrels, triggers, sights, twists and shape of rifling, bullets, compression, crimping, lubes, etc. Knowledge of each subject will give you better understanding of your gun, load and how it comes together, (or doesn't) and will slightly improve your shooting. Almost all the remainder comes down to the one pulling the trigger. A lousy shot with the best equipment is still a lousy shot. A good shot is generally a good shot, even with just fair equipment.
