If you already have, say, .490 or .495 round balls, be aware that if you find a Pedersoli rifle or pistol, the bore might be a bit too tight. I had a Kentucky that would not accept a .490 with a pillow ticking patch. .480 worked fine though. Mine could have been just a one-off, but have heard of others being a little shy of being a true .50.
The Spanish Traditions (formerly CVA) are decent rifles and my St. Louis Hawken is super accurate. The heart of the flintlock is....the lock. the mass produced guns just aren't as consistent in this regard. A custom built rifle from quality parts will be a very good bet, or perhaps get yourself a kit from Jim Kibler and you will have an outstanding rifle that is easy to build and holds value.
I would start with a rifle.