1st - a good lock (siler, l&r, ....)
2nd - a good barrel(Hoyt, Don Getz ,green mtn. Rice, Colerain, or other proven makers)
3rd- then some one who can put them togather ( a Knowen builder will increase the value ) but there are several unknown builders that do Very Fine work look to fit and finsh
Find a club near you who have Black powder/ flintlock persons see what they are using and ask & learn,
they may shorten your learning curve.
Some guys will let you try thier guns, All like to brag about thier KIT&Guns
you might even find a good flinter for sale who's owner is moving up in class(cost) or estate sale
When buying used check for pitting , barrel bulge(run a tight fitting oiled patch full lenth of the barrel feeling for loose/sudden easy-drop of the rod)rust ring gritty feeling near ball seat. funtion of the lock&triggers do they hold at half cock, release cleanly, fire double set and unset...
check stock fit and for damage, splits (hook breaches near tang area) and screw heads abused or sharp.
a well cared for rifle will show expect normal wear but look for abuse or neglect rust ring in barrel(wet patch left to long/ not cleaned after use)
There are ok/good production guns But there might be a great used one(custom/semi)
Kibler's Longrifles in your future if you know what to look for
the Great plains rifle was going near 800+new locks need work ,TC is gone but are still some good ones parts might be hard to get, yes I have a Traditions but it needed lock & trigger work to make it good.
All are fun
Here are some of my custom flinters (Allen martin,Fred Sampson, Cabin creek , Nate Mckenze) and side by side with some production guns Traditions Tenn. Trad Kentucky CabincreekYorktown(custom)and great plains rifle you can decide what makes a good/great flinter