Gregory,
Boy, are you gonna get a bunch of different answeres. I would say that one of the best in a kit is one of the three Track of the Wolf's Hawkin kits. They might be a touch over the 600.00 limit though. Austin and Halleck make a nice rifle. Clear Creek sells the Mowrey rifle and an underhammer also, I believe. Of course there is T/C and Lyman, and Cabella's. Dixie Gun Works also sell a few Hawkin replica's. And of course, The origional Hawkin Shop still sells Hawkins, though at a price a bit more than your limit. You might want to check out the prices of the Northern Rifleman. I once got a great deal there on a Pedersoli. And don't forget Guns America, Gun Broker, and Auction Arms. I saw a almost brand new Lyman with all the extras sell on one of the three for about 205.00 - though it don't happen every day.
Take your time, ask a ton of questions, go to a range where black powder Traditional shooters get together - stay away from them in-liners, they will corrupt you - and talk to them about what they're shootin. Second to shootin, Traditional shooters love talkin about their guns.
Please ask all the questions you want on our forum, you will get 17 different abswers, but most of em are somewhere near the mark. And when you finally get that rifle you have always dreamed of, write us here about it.
Best of luck in your endeavor,
C F