Bart, we can give you a list of things that we’d check out, but it might be a better idea for you to pay a mechanic or surveyor to look at the boat BEFORE you drive three states away. It might save you more money in gas and time than the mechanic’s bill. Floater is also right about the psychology. If you drive all that way, you might be less likely to drop a deal that you really should drop, or hold a line that you really should hold.
Either way, I’d definitely get a mechanic to inspect the motors. If it’s a smaller boat, the same mechanic can check everything else, too. I’d hire a marine surveyor to inspect larger vessels, though (but still get a mechanic to check the engines).
On top of that, I’ll tell you what I look at in your situation, and I’m basing just this on what I’ve seen as problems on boats over the years. I’m obviously not a mechanic or surveyor, and I’m not an expert. I’m a marine insurance agent who’s had, run, and negotiated over several boats over the years and seen a variety of insurance claims. I’d sea-trial the boat (no exceptions), and check as much of following on the water as possible (and where applicable) while someone mans the helm. Check the rest on land. I am probably missing some things, but this should help some. Check the pumps (i.e., bilge, raw water washdown, sinks, heads, etc.), fuel-tank valves, batteries, through-hull fittings, hoses and their connections (looseness, dry rot), primer bulbs, water tanks and their hoses (which burst sometimes during the winter), fuses, breaker panel, electrical connections and switches, electronics, viewable wires, hull for any obvious delamination and blistering, trim tabs, ground wire and block, steering system. If the boat has a head on it, make sure that sucker and its macerator and pump-out are working! That can be a pain to fix. Check for unusual or excessive stress cracks, especially in important areas. Check the motor mounts and nuts. Check the fish boxes for cracks that leak into the bilge. I also check the screw