James,
It's been said on here many times, but it's hard to go wrong with a Chambers Kit. I would recommend one for several reasons:
1) Stylewise, I'm a big fan of Flintlock longrifles. (That's just me)
2) Top of the line architecture is built in for you. You will likely learn certain things about longrifle architecture, after completing a Chambers kit that you hadn't noticed before.
3) Top of the line quality components.
4) Excellent people to deal with
I would recommend resisting the temptation for too much decoration on the first gun. Good lines, (a given with Chambers guns), will carry a plain gun, but poorly executed carving and engraving will detract from the finished product rather than add to it. If you can pull it off well, then that's different, but many people try to hit a grand slam with their first attempt, and end up striking out.
Work within yourself and you'll end up with a great gun that performs as it should.
Just my 2 cents,
Jeff