One of the plusses that the '79 GS has over the later models is it's the only year 850 to have a kickstarter. That is more useful than you can imagine. And sure to stop any ball breaking on base, being a 'real' man to kickstart his bike. Really it's like kick starting a sewing machine if it's all running right
I'm a cheap sort so I'd go with the 50k mile GS. Whether these are miles or kilometers it makes no difference. GS's can run forever and easily go into triple digit mileage needing no major engine work.
I'd be cautious of the goldwings that haven't been on the road for a while. Sitting does no favors to a bike, and things like the tires, rubber lines, and fork seals will all need to be replaced. Adds to the cost of the machine. Being a helicopter mechanic, I have no doubts you have all the skills and then some to keep an old bike going.
Are the bikes ready to run and pass DOT certification or the German equivalent?