Almost any of the modern inline rifles come pre tapped for a scope. Yes they can be scoped and accurate to some hard to believe distances. The traditional sidelock rifles can also be scoped. I would suggest a gunsmith doing this as sometimes additional modifications are needed to make the scope work.

The best caliber out there would have to be the .50 caliber for general purpose. There is a wide selection of projectiles. And they can do anything that any of the other calibers can do.

Good luck into your entry to the black powder field....