It is hard to beat a Lyman Great Plains in your choice of roundball or conical twist barrels. I would stick with the percussion version for starters but one can deal with flinters with just a little more work.
The Lyman GP comes in both assembled and kit forms. The kits are pretty minimal work-wise as rifle building goes.

Brent

What kind of loot could a fella be expected to drop on such a kit?