I don't have a guess as to a fair price for a 1984 4KS w/automatic,
although I've got one. As one of the original owners (wife's car
until she replaced it two years ago), I still drive it as my daily
commuter. Mine hasn't been driven as much as the one you're
considering. I've only got 149,889 miles on the odometer.
But I can answer one of your questions. The engine won't be
damaged if the timing belt breaks on a running engine. Last fall,
while cruising down the Mass Turnpike, my timing belt broke.
Luckily, it was just after the evening's rush hour traffic, so I
was able safely to coast to the side of the road. Besides needing
a new timing belt, no other damage was done to the engine.
'84 4KS (one day I'll go ahead and get a Quattro)