I purchased a crashed r1200gs a couple of years ago. great fun.
I replaced the following:
Main new components I got were:
New forks. Both. - Decided not to muck about with forks. One was stuffed but other was only slightly bent.
They are light tube and I decided it could not be reliably repaired.
New bearings in front end. (Wheel bearings, steering etc.)
Found a wrecked bike in Australia. (I'm in NZ)
From wreckers I got the following:
Front frame. (Cut the frame number off the old one and glued it to new one)
Sub frame for speedo etc
Telelever from beemer boneyard. (3 day delivery from US to NZ)
=> This was from an r1200RT and was actually different to the GS. On the RT the hole for the ball joint is set slightly back. I worked out that it steepened the steering about 1 degree. You change the geometry more jacking up the shock a few turns at the back. It rode great.
I fixed/repaired a lot of components as I was happy with the quality.
Rocker covers both worn through. I got them welded up and painted. They have lost their writing but look fine.
My best tips:
1- BMW repair manual.
2- gs-911. This tool enabled me to reset the errors resulting from the crash. (ABS etc)
I included gs-911 in my budget and have kept it. Went to x-challenge after my 1200 and now have an F800gs.
It was great fun.