RE: Which muzzleloader would you purchase?
The used Navy Arms is definate NO! For what they are asking for the others, there are better rifles out there for less money. Cabela's sells a NEF Sidekick brand new in the box of $159.00 which would be a great rifle.
Check the Auction sites and look for New in the Box rifles. Sometimes you can get them real reasonable.
Perhaps give us a dollar figure of what your willing to spend and what kind of rifle you're looking for and the forum could help you find something...