This perfectly modified 86 just sold for $96k a few weeks ago. I would guess yours would sell for about the same. Modified turbo cars do not bring a premium on resale.
But if its a long hood that resembles a Singer car... lookout!
Modified 1986 Porsche 911 Turbo | Bring a Trailer
So like Adam said "it's worth more". The guy is looking to make a quick 20%-30% off your car.