I think you are seeing a pretty consistent pattern here. I don't have a chance right now to show the details, but here are a few pictures of a door replacement I did. The original door was all metal (door and frame) and made for the 6" thick poles and metal siding, but it was installed badly and completely rusted out. I replaced it with a standard residential metal pre-hung door. I started out by putting in new 2x6 studs all the way up, as tman recommended. I then framed the door opening inside these studs with 2x4s, biased to be even with the inside of the 2x6s (because I was going to finish the interior. I then installed the pre-hung door, with brick mold trim, into the 2x4s. This left the door recessed, but when I put new siding on, I trimmed it out with metal trim inserted into the gap between the 2x6s and the brick mold. Worked out very well. I won't be back home until this weekend but I will try to get some detailed photos to show how the final trim worked out.
It is the standard "L", "J", and drip edge trim that is available from the metal siding suppliers. I replaced all the siding and trim after I replaced the entry door and added a new second overhead door.
IH Farmall 656 gas/ IH 240 Utility/ 2, Super C Farmalls/ 2, Farmall A's/ Farmall BN/McCormick-Deering OS-6/McCormick-Deering O-4/ '36 Farmall F-12/ 480 Case hoe. '65 Ford 2000 3 cyl., 4 spd. w/3 spd Aux. Trans
I used the vinyl brick mold trim, attaching it with Stainless screws, when replacing the wooden stuff, after it started rotting within 3 years.