Hey Justin, Welcome
So I moved this over to smoothbore since that's what you're ultimately looking to get.
Now I'm going to suggest what I would do. There are several things that can be done to meet your needs. You didn't mention any particular style nor historic era, so I'm going with you simply want a black powder long gun, for targets and hunting, being very versatile, and ALSO not being too hard on the beginning budget. The following is only one method. Other folks will suggest other ways, and if you narrow down what you're wanting I'm sure there will be more detailed answers and options
So I'd look to find a good looking rifle, be it a full stock or half stocked "plains style" rifle. In fact you are likely to find such in a plains style simply because of CVA, Leman, and Thompson Center making so many of them. Find a used one with a bore damaged by improper care. Likely not able to shoot a round ball well because of the rust, BUT that should bring down the price of the used rifle. The lock working right is the main thing in this first scenario.
THEN what you do after getting the rifle, you send out the barrel to a fellow named Bobby Hoyt, who will ream-out the barrel to a bit larger caliber, removing the damaged portion of the barrel, and returning to you a newly made "smooth" rifle barrel. Voila! You are now ready to shoot. Flint or Percussion is your choice.
Typically, folks have old rifles with "roached" barrels reamed out to the next popular caliber so a .45 becomes a .50, and .50 becomes a .54/.55 (.550 is 28 gauge), a .54 becomes a .58.
LD