Is it the original GS or the "new" GS ?
It matters concering the shaft - one or two U-joints that may cause trouble... You need to check them! Kind of expensive to fix.
60 000km should be ok concerning the gearbox, but put the bike on the central stand. A bike-table/lift is a good place - so you can strap down the fronwheel and secure it to the ground.
With the rear-wheel free in the air; engine running at 3000rpm - start shifting up from 1st to 5th - use a long metal rod of any kind (stethoscope) and place it at the front and rear of the gearbox where you have the bearings. Listen for bad ballbearings and sprocket teeth - if it sounds ok, then you could be fairly sure that it is ok. If it sounds like stone crushing... the gearbox must be overhauled at once (costs a lot).
Pull up the dip-stick - look at the engine oil, feel it, smell it, and taste it. If burnt, coke, harder particles, petrol, etc - a major engine overhaul may lurk around the corner.
Beware! If the engine oil is in too good shape -ask to see what the owner poured out "last week"...
Same goes for the gearbox, shaft and final drive - check the magnetic plugs for any metal - if you see flakes... it may get expensive...
That is a 8.2:1 version. 8.5kg is minimum. Above 10kg is very good. If around 9.5kg it is quite ok.
Need only to put a multimeter across the battery - it should read about 13.7V or better. If more than 14.4V - someone have already tuned the system for you.
It differes also between driving-lights on or all lights off - you may get 14.4V with no lights on at 3000rpm and up; and still it drops to 13.7 when the lights are on... No probelm to fix -
BMW just tunded the system a bit low.
Check the forks and stearinghead for play (should be stiff).
Check the wheels for play in the wheelbearings (should also be stiff).
The swing is a bit harder to check - but you can feel a play if the bearings are bad.
Expect the rear shock to not have been overhauled - anticipate that as a first major expense!
Check the forks for leakage. You can use automatic gearoil instead of fork-oil. I use Omega 699 (5W/20 - non foaming).
The hoses between the cylinder heads and carbs can you expect to be cracked and leaking air - age.
Chekc the top of the carbs - the steel-plate gets loose over time and leak air... tricky to fix.
The pushrod tube grommets are expected to have reached their age-limit... expect them to leak oil. Not too much job to fix, but it you do you may want to install the 55hp pistons (1978 R80 + pistonrings for nicasil barrell); runns better with those pistons.
There is a bit of querstion mark on the oil-pump O-ring... you may expect to have to change that in a short while... not much job, but costs a bit... the pump is rather inexpensive though -
I have most likely forgotten quite a bit -
The price sound ok from a Swedish market point, perhaps you could get it down to about 2200EU which would be our "proper" market value - other countries have other market levels -
I'd say, by your description, they you have a good object at hand that should be fairly safe to invest in -