Get the .54, then you'll see which size ball and patch thickness it likes best. That's half the fun right there, getting to know your rifle.
It will shoot either a .530 or .535 diameter round ball (RB) and patches can be anywhere from .010-.020
For instance, my .54 likes a .530 RB along with a .018 thickness patch however, every rifle is an individual and it's up to you to determine which is most accurate. If it is available in your area, (you have to ask for it at the counter of the firearms section) use a traditional type black powder, such as Goex, Swiss, KIK, etc. A new barrel will also likely need to be shot in, to remove any sharp edges from the rifling left from when it was bored, these tend to tear up patches and affect accuracy. You can also smooth things out with steel wool or other methods. Hope this helps, good luck and let us know what happens.
BTW, I'm going to assume that you've chosen the slow twist barrel for shooting RB's, so no need to try conicals in it.