From what I can tell, I would call that a near perfect motor based on its age. Around here, most people would ask anywhere from $200-$1200, depending on how much homework they have done on the motor. The $200 ones get sold right away, the $1200 do not. I would say a fair market price in my area in peak season (like right now), would be about $650-$800. Personally, I would not buy it for more then $500, but that's simply because 1. I do not need it and 2. I always like a bargain and I am not afraid to tinker on a non runner/rough runner to get it running right. Someone who just wants a good reliable 25hp motor without breaking the bank i'm sure would pay $750 for it right now.
Don't be offended by an offer of $350 on it, honestly that's about where I would want to be into it if I was going to buy it--but again, I'm not really the 'target market' for it.
But good for you for wanting to keep it! You will never own another outboard that will mean as much as that one!