I have 585 hours on each of my 2002 F-115s. They are still running strong and with the exception of where I've dragged the skegs through a sand bar, the motors look as good as the day I got them.
In general, most of the boating public puts less than 50 hours a year on boat motors. Commercial folks can put up to three or four thousand hours on a motor before they think about trading them. Motors that get run every day (like a water taxi) with a thousand hours are probably better maintained and in better condition than one that gets 50 hours a year and has 200 hours on it.
Any rust on bolt and screw heads is an indication that the powerheads weren't rinsed with fresh water a couple times each season and then sprayed with T-9 Boeshield (or something similar). If the owner didn't do that simple task, I'm left wondering what other preventative maintenance has been forsaken. If I found rust under the covers, I'd heavily discount my offer.
Neglect is probably the greatest threat to a modern motor.
Dave
aka