I had a 70s Double Eagle 17.5, with volvo straight 6. Smaller boat, smaller engine, same in principle.
I loved that boat, but it was a disaster financially. Call it beginner mistakes...here's the things I'd look out for.
Inboard motors generally: if you are mechanically capable to fix on them yourself or you have access to cheap mechanics, they are good. Old ones are simple motors that a backyarder can work on. But if you need shop assistance, they can be a nightmare, because if you want to replace the motor you're into $1000-2000 in costs to pull and reinstall without even considering the cost of the motor itself. Add in the inevitable problems with broken this and that, and it gets exceptionally expensive.
Volvo vs Merc/Other: there's a fair amount of volvos out there, but there's tons of Mercs and they are way cheaper and easy to get parts for. I tried to repower with a volvo motor, only to find out the blackbox/ignition coil was gone...$1200 part for a $1000 used motor, couldn't find it used. That's a kick in the butt. Second, found out the exhaust manifold wasn't working right -- finding them used is not really an option, it's the first thing to go because of salt water corrosion, so you're looking at $2000 for that. These old motors can get expensive when they start to go...lesson learned is better to buck up for a newer one than try to mickey mouse around keeping old one alive.
FWC: you definitely want it fresh water cooled. You can add this on after-market, so it's not a deal killer if it doesn't have it.
Hull: look for soft spots, if the floor is spongy, then the hull might be full of water. Wet stringers will mean a rotten hull, which means you don't want to be anywhere near it unless you are looking for a serious project. Bang on all wood you can find and listen for hollow sounds (dry rot) or exposed wood (is it wet? has it taken on any water in past?). For example, look at all exposed wood around gas tanks, motor well, anywhere that you can access below deck. Someone told me corners are the key, that's where the weak spots are, and where problems will start.
Transom and bilge: get down in there and inspect anything you can see. Is the bilge clean and cared for, or dirty and possibly hiding problems? Bottom of the transom in motor well and where batteries are stored on boat gunwales are the main spots to check.
Outside: rap with your knuckles or a rubber hammer every few inches, listening for hollow sounds or anything different that might indicate hull isn't solid. I'm no expert or surveyor, but I think you'll be able to hear trouble spots -- and if nothing else, it doesn't hurt to have had your hands over the whole hull BEFORE you bought it. If you do find problems, a surveyor might be $300 well spent (I don't know what they do, but when I got my current boat inspected, they reported results from a moisture meter, that can tell how much water is in the hull...good tool for hidden nightmares).
Future options: if the hull is super, but motor goes, you can consider plugging the transom hole, putting on an aluminum pod ($2500ish), and getting an outboard. Advantage to this is you gain all the deck space of the motor, with the high transom right to the stern.
The end of my Double Eagle was fixing the floor (solid hull), attempting to repower and failing, and hitting the wall financially and family tolerance....sold it for pennies on the dollar to a guy willing to take on a project, and I hope to see it out there someday (but last I talked to the guy, it is sitting in HIS yard waiting time/money to get it done...
Good luck!
