With your budget.. lonewolf made an excellent recommendation. I personally no longer do any business with D&R Sports for reasons I will not discuss,but that is my personal opinion. Just be careful dealing with them. I had good luck with them for a long time, but the last transaction turned sour and it is THE LAST TRANSACTION.

The Trade Rifle is a good choice in .54 caliber. It will shoot a .530 or .535 roundball with excellent accuracy. My Trade Rifle shoots 90 grains of Goex 2f and a patched roundball with excellent accuracy. Also it has a 1-48 twist and shoots powerbelts, maxiball, and REAL conicals very well. With the hooked breech design, it is easy to break down and clean. Lyman has an excellent stock to hardware fit/finish (at least in my rifles). The stocks come in need of a little more attention for the owner to decide how dark they want them. So you can seal them with wax or make them darker with a little boiled linseed oil or tongue oil. The Agate that comes with them is excellent also, but you will need to get some of the Thomas Fuller Black English Flints sooner or later.

Another great choice is you do not mind a little more weight and length is a Lyman Great Plains Rifle. You can get them in a 1-60 twist for roundball shooting or a 1-32 twist for conical, sabots, powerbelt type of shooting. Midsouth Shooters Supply has some excellent prices on them right now. In fact I should buy one instead of going to their sight every day. But I have enough flintlocks (if that is possible). With this rifle you can change out barrels to the different twists and have the best of both worlds. Also I believe Green Mountan Barrel Co makes a barrel for them.