Hello, good luck with your purchase of GS, most of the points to look out for have been covered by the others.
Make sure you have a good test drive, not just round the block, spend at least 20 minutes driving and go down the motorway and do 70mph. Check for wheel wobble especially when braking.
Find a car park and put on full lock both ways and listen/feel for any knocking through steering wheel for ball joints.
All the best
Paul.