By the way, you asked about price. Here's the NADA value for the 1980 model:
1980 O'Day O'DAY 25 Standard Equipment, Boat Value, Boat Price & Craft Specs | NADAguides
It's $4300 - $4900. But the higher number is the "average" retail, and, I'd guess that that corresponds to boats in "average" condition, and doesn't include the engine. If the boat actually looks like the boat in the ad, and if she has been as well maintained as she appears, $5500 isn't bad. I'd still see what I could do about getting an even better price ($4,000-$4,500 initial offer) but that's me.
Good luck and let us know what happens!