[Hobster]

WizardGuss has covered most of the major items.
You will also want to check the engine lower unit. After a sea trial
(don't even think of buying without one)
After the engine has been in the water running, unscrew the lower plug in the lower unit. Then slowly unscrew the upper plug where you put in the fluid. If you see any water come out the bottom or the fluid is milky white, screw the plugs back in, say thanks and walk away.
If you are not sure how to do this, bring a friend or mechanic. You don't want all the fluid to drain out.