Right off the bat.......I'd say it sounds like a pretty good deal. Since you're in tight with the shop guys, I doubt they'd try to shaft you.

You're right on track with what you considered for inspection areas. Look down the bore, inspect for rust, make sure the lock action is smooth and strong. Also, I'd unscrew the nipple to make sure there is no corrosion on it or the receiver threads. If possible, see it you can fire a couple of caps off with it too, that way you can verify that the lock is striking well and that the flame path is open for good ignition.
Beyond that, checking it out is no different than any other used firearm.

Good Luck & merry Christmas