First, I wouldn't consider a dirt bike an investment. Second that is a 1981, not a 1982. I have a 1982 GS. The 1982 models are monoshock.
GS is more rare than the MX version. The real difference is just the ignition and the gear ratios, for trail riding the GS is going to have better gearing. Ya, you can change the gearing however you may like the GS gearing, it depends on how you ride. Replacing the gears is easy you can even buy a new set direct from Koestler.
That bike in that condition and the rarity of the GS model the asking price is high but still reasonable. However, I am not sure that it would or should go too much higher than 4000 Euros. Most 1981's recently sell around that range, so since it's a GS and rarer it may be worth a little more. It all depends, the prices for Maicos keep going up so who knows what someone is willing to pay for that right now.