Ignore it all. They all do everything you need and more. If you want any "cool features" they:re probably more on the mapping side.
ALL of the hyped sonar features are for fishermen.
An auto correct of "good transducer installation"
Anything will fit the purpose you described. Get in a store and play with some to see what you like, and choose based on that.
Chart plotters have maps that are likely quite handy for you. On the fish finder side almost anything is fine if you don't care about fishing. All that will really matter is getting a foot transducer installation.
The bathtub curve never misses. New stuff is more likely to break than slightly older stuff that has had a time for the weak points to fail. Once those initial quality issues are worked out and the lemons filtered out...you get a long period of relative reliability. And finally stuff starts...
I'd rather not pay the 5-10x premium it'd take to get into a similar new boat, and save those 10s of thousands of dollars for other things, but that's just me.
Yup the cost of doing it right is not that much different from the cost of doing it wrong, and it doesn't set you up to break things.
Of course you might run into more that needs doing.
The bane of old fiberglass boats and the type of rehab they can require.
Aluminum, if the hull is sound you can more or less always make the boat operational at a reasonable price. You might choose to go beyond that on a boat you're gonna keep (and a lot of us do) because you're still saving...
That's not to say you can't use a partially completed project...but if you can't afford to replace floor and foam you don't rip it out - especially since the "cost" to do it right is far more time than money.
Other than being slightly more difficult to source props for "wrong" rotating engines I don't see where there would be any difference in a single-engine config.
How much clearance do you have currently? I think Watermann's Islander had been hacked to take a 20 - not in a manner I'd recommend but presumably it worked? I'd probably try to hunt down a 25 though.
