Agree with sdporter.
I would ask for detailed pictures if buying from an online seller and wouldn't let a "k" at the start of the serial number (normally the way to for sure tell if it was sold as a kit) worry me one bit if it looked good.
Besides normal outer appearance, look for inside bore condition (deal breaker unless willing to have it sleeved, replaced, or rebored), wood fit to metal (don't want big gaps), ask if the wedge pin is nice and tight if a kit gun, ask if set trigger and main trigger work as designed, ask if adjustment screw between the triggers is there (these often fall out, blue Loctite as soon as you get it), ask if it catches and stays at both half cock and full, and look closely around the nipple and in hammer cup (normal areas that are missed/neglected during cleaning). Oh...and ask if it is unloaded! You'd be surprised how many are loaded / have something stuck in the barrel (they should know how to check with the ram rod).
As far as 45 or 50, if just for deer take your pick. I'd probably go with the 45 since they normally eat less powder, especially if wanting to buy and use real black powder.