In-lines are fine if you find one you like. There is however no physical reason why a good sidelock rifle can't shoot just as well. Musket caps are less powerful than 209 primers, and #11 percussion caps are less powerful than musket caps, but that's a reliability issue rather than one of accuracy. If I really shoot a lot, on a given day I might have one or two misfires using #11 caps, but then I don't do any cleaning of the nipple 'till I get home. I could also beef up the mainspring without any trouble, just by adding a washer to the end of the coil.
Good sights and a good trigger help also, but do not affect the rifle's inherant accuracy-- just the ability of the shooter to use it well. If you're a decent shooter, and you end up shooting massive groups, something's probably loose, or your loads are way out of the zone your rifle likes.
With my Lyman Deerstalker I could keep up with most people shooting a standard Mini-14. I know because I have both, and I can get 2 to 3" groups with either at 100 using the irons. Truth be known, the Military specs for the M-14 (crica 1950s) required a five-shot group no more than about 6 inches at 100 yards. That's a modern gas-operated rifle. With a little tweaking and careful load selection, most any current manufacture muzzleloader should do better than that unless it's defective in some way.
Cabela's has their Blue Ridge rifles (I think from Pedersolli) in both flint and percussion, several bore sizes from small to large, and they've gotten pretty good reviews (and they're oh so pretty). You can go from this link and find the reviews from there. Some of the commenters know very little, or they're doing the review without having fired the rifle, but there are at least a couple of them who really shoot and they know their stuff:
http://www.cabelas.com/cabelas/en/t...index&indexId=cat601141&rid=&_requestid=79812
Cabela's also have a "Kentucky Rifle" that is a better bargain, but it only comes in .50.
The TC Hawken is a good first front-feeder, but again, no small bores that I know. Green Mountain barrels can sell you a barrel in just about any bore size, for just about any rifle, however.
How many shots before fixing a flint? Depends on the flint (and probably the rifle too) but I'm not a flinter (yet) so others will have to comment on that. See the stickys for this section and there you'll find a lot of good info.
Both the Knights and the TC in-lines have good reputations.