Type of firm/place to contact to have house features pre-finished?
Sounds reasonable to out-source it, but you will still need to do a final coat on the casing after you install (door should be fine if you are careful).

To "do it right", you need to fill the nail holes and corner joints and sand, and then run a bead of painters caulk around the casing to make the cracks disappear, then paint again.

I just did the caulk on the inside (casing-to-jamb) not the outside (casing-to-wall), and didn't paint very far onto the side of the casing where it hits the wall. This way, you get a nice sharp line at the wall without a lot of hassle.