Hi There, the life of an engine is generally down to the history but it is hard to tell still as paper history is only one part of the story. The previous owner may have serviced the engine by replacing plugs and oil but has he flushed the engine through after use? Has he been running the engine at high revs for long periods?
Over 300 hours is a fair length of time but i would be inclined to physically check the engine rather than go by hours. Taking off the rocker covers is a good start as you can see if the engine has had regular oil changes, are the rockers and valve tips worn? Are the valve seals working ok?
Its allways a risk when buying a boat as the owner will tell you what you want to hear for the sale, these engines dont cost fortunes to overhaul anyway as a worst case scenario but physical checks are the best way to be sure. Alot of the marine engines i re-build have issues in the water jackets where they havent been flushed through regularly and you get a build up of rust in the water ways.
I hope this helps, if you need any info or help let me know.