Replacing exterior door--hang it myself vs. pre-hung
I have a home built in 1951 with a walkout basement. I've just finished framing and insulating the block walls in the basement, which of course has added several inches to the wall thickness. The door leading to the garage needs to be moved back flush with the interior side of the wall, and I'm trying to decide the best way to do this. The door is original, a heavy wooden raised-panel door with a half-window that is single pane. There are jambs nailed into the ends of the adjoining block walls.
At best, the wood door with glass has an R- value of 0.5 to 1, and feels pretty cold in the winter. I was considering replacing it with an insulated door (the glass is also a modern code violation, but that's not worrying me at this point).
If I keep the door, I need to replace the jambs with those of proper width, and rehang the door, though I've never done that before and have heard it can be a chore. A pre-hung door would solve that problem, though that would add a wooden threshold that currently isn't there. I've got a lot of heavy woodworking equipment in the basement and its nice to be able to roll it in/out of the garage as I do at present (the basement and garage slabs are flush).
Does it make sense to replace the door with an insulated door?
If I went with a pre-hung door, would it make sense to cut away the threshold after installation, or would that weaken the frame in some way?