Obviously that it starts runs , is clean , floors not rotted (spongey) Dip stick the Jet (small hex stick) The Jet should have a very small amount of clean 90 wt gear oil showing ,NO GREASE (comon to grease these when seals fail) Boat should snap up on plane and accelerate smoothly , cables are a constant expense on these boats , all should be in good condition and work smoothly I could go on but call if you have questions I'll be in till around Three today then I got a Keg calling me  Tom

Some of the things I think you need to be looking for...

PUMP: Damage to the impeller, or housings (if everything looks ok visually, you still should have a shop double check after purchase).

ENGINE: Rust in cylinders  .  With the engine out and intake/exhaust off you will be able to see some "things" but that would not be enough for me.  Take some tools with you just in case.  Remove the spark plugs, inspect the plugs and (if you can get a bore scope) look into the cylinders.  Also remove the oil pan drain plug (ask if empty first  ), to see if there is any milky residue.

HULL:  Look for soft spots in the floor, cracks or damage to the bottom, broken bunks on the trailer.  Take a mechanics mirror and flashlight to look into the tight areas.  Look at the upholstery wood for deterioration.  Move all the controls to make sure everything still moves.

Wife/GF:  Take them with you, because if she doesn't approve now, she never will 

Good Luck