There are 3 items in a flint lock build that there really isn't a place for mediocre quality; the barrel, lock, and trigger installation / mechanism. You could add the sights in to that as well.
Most barrels from the major barrel companies like Rice and Colerain are of good quality, so that really isn't an issue. Some times you can get a deal on a used (or new but unused) one off of Flea Bay or yard sales. Simple pinned triggers can be every bit as good as the more expensive and complicated trigger assemblies, but when it comes to locks, yes, you can make a lesser quality one work, but be prepared to spend a fair amount of time adjusting and tuning it to make it work as well and reliably as a premium lock, like a Chambers. That's your fire control group.
The rest of the stuff (stock, trigger guard, butt plate, patch box, pipes, muzzle cap, ram rod) doesn't have much to do with accuracy or reliability, but sure adds to the aesthetics of the package.