As Ronald Reagan always said, "Well..."
It depends on what you are going to do with the rifle you buy.
Sounds like you are interested in hunting in which case your rifle should match the game you are intending to bag. It is also true that depending upon which state you are hunting your season may change depending upon whether you go Flintlock or Percussion. In some terrains a long reach is important. The folks at the store can sell you the right rifle by asking you about three questions. You might consider buying a used rifle (in good condition) because a lot of times the seller will throw in a bunch of stuff.
You have to have the rifle, powder, ball, caps, patches, cleaning and lubing supplies. You will need some simple tools which should include at least a nipple wrench. I recommend a good supply of Newcastle Brown. Don't forget the safety gear. Eye protection is absolutely indispensible. The hunters I know do not use hearing protection but they always answer every question with, "Huh?"
I would spend a good bit of time reading some of the posts on this forum using the "search" function. There are plenty of good books out there. There is also a lot of good info elsewhere in the net.
I may be the first person to respond but I am far from the most knowledgable. I do not hunt and never did. I think that all deer should be left to peacefully grow into old age so they can move into a deer retirement home and be taken care of by all those deer nurses. I got rid of my last long arm about ten months ago and I have no desire to ever shoot anything with a barrel longer than ten inches.
I will give the other, better informed guys an opportunity to weigh in, but let me close by making sure that I mentioned the Newcastle Brown. Don't take it along when you are shooting. The bottles rattling together will scare off all the deer. I think all deer use hearing protection when they hunt because their hearing is real good. No, save the Newcaslte Brown for when you come home to clean up the rifle.