as a building contractor I have seen many homes in which people started to build themselves and then could not afford to finish. However none that have sat since the 80s without someone buying and finishing them.
it would be more interesting to see more information about the house

Finishing the inside of a house usually takes 75% of the money to build it, and 90% of the time.

I'm working on a water damaged house right now. Sad thing about it, was that two AA batteries caused $30,000 of repairs. The toilet bowl cracked during one below zero snap, no one was in it at the time. But the water was left on, and this toilet was on the third floor. First floor hardwood floors were shellacked, second floor, urethaned. The second floor hardwood survived, not the first.

Paint, plaster, new hardwood floor, one new bathroom, one new kitchen, and months of busy work is getting this house together. Fixing the screwups other contractors did is what I hate. But that is also why my customer has put me in charge of this job.

All good advice I'm sure but considering were you live (small town in the west) to me makes it less odd that if it were in Ohio for example.
Know what I mean?

Yes, I know what you mean, but apparently nobody ever lived there. Who knows what the plumbing is like? Is it electricity, propane, gas? Wood stove or no. What would an inspector see?

I can't remember if there is a wood stove. That is important.

Out of curiosity, how much land was being sold with the house?

For a house to sit vacant and unfinished for 30+ years and still have an asking price of $85,000 probably means somebody other than a bank owned it. But if a bank or real estate company actually does own it, maybe the $85,000 is mostly just the value of the land rather than the building itself?