I am considering a 95 or newer nautique but am finding only boats with high hours are affordable, at almost 800 hours. What should I do and how can I determine if the engine has been taken care of and will not require a major overhaul or major repairs in the near future ?
Ask for service receipts, if they did the work themselves you might not get any. But if they didn't, then they might not have had it maintained properly, and if they did that is a good sign.
have a mechanic survey it, check compression, etc, you can have oil analysis done to it but i'd only do that if you were sure you were buying it barring major issues.
If the engine and trans oil was changed regularly then 800 hrs is much less than 1/2 of the expected engine life. 90's Nautiques with fuel injection have overheat protection that helps insure that the boat has not been run hot. A compression test, or a leakdown test is always a good idea though.