Buying an old Lincoln (78 Mark V) - what problems can I expect?

Hello:

I am considering the purchase of an old Lincoln: 1978 Mark V, with
@95,000 miles. The body is decent, not perfect. Engine in good running
condition. Brakes, exhaust, power accessories all okay. I am not a
collector - just want a car to get around in, and not too worried about
the gas mileage (this car gets about 15 mpg). The person who is selling
it is a mechanic and is quite honest. He's asking $800 for the car.
I've been warned by others that I may be getting into an expensive
repair situation with this vehicle if something goes wrong. I'm not
that concerned about the routine stuff (though I guess the parts are
going to be hard to find, correct?). But I wondered if anyone in this
group would know whether any big-ticket repairs could bite me down the
road.

For example, the transmission. Can I expect it to last a while? Say, at
least another 20k?

What about power window motors and parts? Can't they be a pain if they
break? Are Lincoln's any more or less reliable than others? Is it hard
to get replacements?

The AC works in this car, and the seller said it's a strong, reliable
compressor. But how about the heating/defrosting system ... can I
expect it to last, or does the age of the car automatically imply that
leaks will develop in the core, hoses?

The other thing that came to mind was the steering and suspension (ball
joints, tie-rod ends, wheel bearings) - do such components wear out
with age, just sitting there ?

Regards
Al