What I am seeing now is that most of the "Cheap" 850's are cheap for a reason.
A $10k ebay '91 850 will probably take a minimum of $5k and a lot of Patience
to make nicely driveable again. If you don't put the money and patience into it upfront it has a good possibility of turning into your worst nightmare of a money pit.
Radiators, expansion tank, hoses, heater valves and aux pump, water pump and thermostat, fan clutch, idlers and belts are either at or beyond normal age failure points.
Hitting the temp red zone buys a dead engine.
Keeping them all working makes up to 300K on the engine before it wears out. Stop Leak will let it run long enough to get it off the truck at your end but not much farther.
The list of suspension parts that are no better than marginal at that age could be pretty long and $pendy too.
Electrical gremlins in a multi-computerized car are a real trip without support.
Remember the simple electrical system on the VW?
This is a '91 850 set of schematics: http://wuffer.net/MyFiles/ETM91.pdf
That said, if you buy an "Enthusiast car" with everything done on it and good for a lot of trouble free driving you are going to part with about $20K to $25K and grin from ear to ear while driving.
I love my 840 and wouldn't trade it for anything but I'm not sure that you would have the money or the patience to keep an 8er running.