Trad Hunter:
I'm happy you decided to include Black Powder Muzzleloading as a addition to archery. I would suggest the two compliment each other in that both require real hunting skill on the hunters part, and a successful hunt with both has more meaning than using modern high power firearms.
Here are a few thoughts about your question:
You want a rifle. The smooth bores musket style guns are rather expensive and (some would say) not real accurate beyond 40 yards.
First off, I would suggest that you choose a rifle which shoots round balls. My reason for saying this is that as with any weapon, to shoot it accurately you need to shoot it a lot.
Round balls can be bought for as little as 8 cents each and if you really get interested in your gun, round balls can be cast for less than 3 cents each. This basically says it won't cost much to get good with your rifle. Round balls also need a patch. If you buy the patches they cost about 5 cents each, but they are easy to make yourself at almost no cost.
All of the In-Line guns have fast twists which will not give good accuracy with a hunting load and round balls.
The In-Lines require the use of a sabot and (usually) a jacketed bullet. This can raise the price for just the bullet/sabot to the 20 cents to over $1 per shot depending on the bullet/sabot you use.
I would suggest your new rifle should be a percussion style.
They are about the same ignition speed and reliability as a modern rifle, the only difference being that you have to place the primer (cap) on the nipple.
As much fun as it is to shoot flintlocks, they are harder to get used to. If you really like blackpowder shooting, you can buy the flinter later on.
You will see several posts here which maintain that a .40 caliber is sufficient for deer but I would say you really need a .45 or larger caliber gun.
The .45 is more than adaquate for deer out to 50 yards while the .50 does a great job beyond 80.
A lot of people choose the .54 caliber and it is IMO capable of taking anything from a moose on down. It is a little much for turkey and small game though.
When all is said and done, I would suggest the .50 caliber.
As for actual rifle choices, if your just playing with the idea of shooting black powder, the CVA sidelock guns will work well and they are relatively low cost.
If your a little more serious and want something with higher quality, go with Lyman or Thompson Center or Traditions sidelocks.
If you agree that round balls are your choice, buy a 1:48 to a 1:70 twist. Do not buy the 1:36 or faster twist.
If your buying new, enough said, but if your thinking of buying used guns, pay special attention to the condition of the bore. Black Powder fouling will rust the bore and some people don't seem to know this. This results in a lot of guns with rusted bores, and rust pits in the bore can tear up the cloth patches which are required with round balls.
As for questions, do not hesitate to ask as many as you want. That is what this forum is all about. Helping others learn about our great hobby (in all of its areas) for most of us only adds to the pleasure of black powder shooting
Welcome to the forum and to Black Powder shooting. Your interest in both place you well ahead of the crowd.