Boats
Are
Not
Expensive.
Marinas are expensive. DIY boat fixing is super simple and no more expensive than doing the same repairs on your Miata.
The name of the hull does mean a lot. Just like with RVs, there are boats that are designed for the people who go boating three weekends a year, and there are boats that are designed for a quasi-professional fisherman who is on the water every day for 8 hours. Bayliner is an example of the "looks pretty on labor day, but won't hold up long term."
Some that are basic floating Yugos made with hopes and glue; Bayliner, Maxxum. Some that are fine, middle-of-the-road Chevys; Sea Ray, Tracker, Nitro, Ebbtide, most of the common brands. Some that are the well-done W124s of the water; Wellcraft, Grady White, Baja, Checkmate, Carolina Skiff.
My last one was a 19' Baja. Bought it as a complete basket case for $1100. The floor stringers were so rotted they had grass growing out of them. I stripped it down to a completely bare hull, including taking out the engine, gimbal, and outdrive. It was truly a bare hull. Three weeks later it was on the water for under $3500 total investment. Powered with a 350 I built with off-the shelf automotive parts, it would hit 65mph, and it would do it with supreme comfort and catch everyone's eye. Nice looking boat, great performance. My current one is an 18' Ebbtide bought as a ready-for-the-water boat for $3500, and I think the aluminum I-beam trailer was worth more than half of that. I put $130 in parts into it and four years later it still fires up every time I hit the key. Still a solid boat, but with the 3.0L 4 cylinder, it's no performer. I also notice it is not quite as brick-like as my Baja. The Baja felt like it was carved from one rigid chunk of fiberglass.
The Sea Ray you found might be a good bet. The Ford in there is nothing special. Snag a random longblock replacement and some gaskets and swap your marine pieces on to it. What brand/type of outdrive is it? The chances that the Ford is a reverse rotation is extremely slim, but worth checking. By the 80s, the only thing reverse rotation was full inboard ski boats, and even they started getting rare by then. The secret is finding the boat that only has DIY things wrong with it.
Solid floors. Look for soft spots. They will get soft. Boat flooring is usually marine plywood with a layer of glass and epoxy. They will eventually get water in the wood and rot. Doing a new floor yourself isn't the end of the world. Having someone do it will cost a LOT of money.
Good vinyl. It doesn't matter if you're re-upholstering boat cushions or a LaZboy recliner, it's not cheap and sometimes not easy to DIY. Flat cushions in a bowrider are easy. Captain's chairs, not so much.
No cracking in the gelcoat. Look closely. Old gelcoat will eventually give in to UV and get hairline cracks. Once that starts you're looking at the possibility of re-gelcoating before water can seep through to the core and rot it. Gel-coating is another one that isn't easy to DIY and expensive to have done. In a pinch, a good epoxy paint will solve the issue.
Good body work. I'm talking like the gauge pods, the windshield frames, and anything unique to that boat. Being that old, you can't just order a new gauge pod, you have to use what it has or fab your own if it's broken. 40 years of someone's knee hitting the dash might give you trouble. Don't worry about gauges. They're the same protocols as automotive gauges so they're cheap and easy to replace.
Ask when the last time it had a shift cable and bellows kit. The outdrive connects to the boat with a rubber bellows. You get about 10 years out of one until it tears and leaks water (and damages the upper bearing if you don't fix it right away). The kits are $150 and you can DIY, but a Marina will charge $1500... because marina.
If you bought an old boat and it needed all of the above, you could spend a few weeks and DIY it all for $500, or you could take it to a marina and they would charge $5000-6000. That's why people think they're expensive.
Look for a Seloc manual. They are the Haynes Repair manual of the marine world. Get one for whatever engine/outdrive/outboard you end up with. Nothing about working on the mechanicals of a boat is any harder than working on a Caprice or a Jetta. In fact, it's often easier because things aren't shoehorned into a tiny compartment flanked by suspensions and body work.