No scope, conical shooter, and reasonable price sounds like one of Doc's Whitetail or Bisonkits to me. http://www.whitemuzzleloading.com/prod04.htm No barrel interchange and no .54 right now(for .54 I would do just as Cayugad has suggested). Doc might even treat you a bit better than the website pricing.

Clean powder? - probably the American Pioneer Powders or the new Goex Pinnacle (mixed reports on them so far as results, but they are definitely cleaner). I've not tried them but my understanding is you should get the 3F granulation. Pyrodex P is cleaner than RS but still nowhere near "clean", however some absolutely fabulous accuracy to be had from that powder in Whites. Triple7 is easier to clean and though I wouldn't say it is cleaner, it is a bit easier to clean and is less corrosive. Triple7 has its own set of problems with crud rings and sticking breech plugs often encountered.

If you insist on having barrel interchange - the Encore is reallly the only quality choice with readily available barrels. One barrel could easily cost as much as one of Doc's kit rifles though. To achieve a degree of interchange with the NEF (also a good rifle) you first need to buy one as combo or centerfire to get the SB-2 action. That action will take shotgun, muzzleloader, and centerfire barrels - the SB-1 used on the standard Hunstman and some others will not. A problem with both the NEF and the Optima is there is no provision to easily clean the firing pin assembly. The Encores do allow cleaning of that area and it IS necessary.