Thank you for all these answers and valuable tips. This helps a lot in identifying the real value of this arm.
The seller ended up asking too much money. We offered twice the value of 'scrap metal' and then some but that was clearly not enough for the seller. So we've ended up not buying this thing. Unfortunately I must say because it surely would have generated a lot of fun playing with it. I guess we'll find another opportunity.
I understand most of you are thinking about running production and making profit/money out of it. This definitively is not our goal. After some experimentation this thing would eventually have finished its life as 3D printer or milling machine or something similar. Our 'team' is crazy enough to build/find some new drivers for those servo's should the old controller not be fully functional, but we would at least need some base to start working from and reverse engineer.
That said, if someone knows about an old industrial robot (no need for a big one) that gets scraped not too far far away.... contact me please.