$13,900 for a car with 160,000 miles!? Typical high end
boutique car lot price. Unless that car comes with receipts showing all wear and tear items replaced, the price is insane.
There was recently a beautiful '87 S4 on Craigslist near me for around $8,000. It was all up on maintenance and had ~80K miles....although it was an automatic.
I would look at buying from an individual/928 enthusiast, and when you find something you like, get a pre-purchase inspection done.
This is what I'd expect for $13K
1989 Porsche 928 S4
Or this for far less
1988 Porsche 928 s4 for sale