It doesn't make financial sense to repower that boat with a diesel, it's a big upfront cost and little chance of getting it back on resale ($4K boat + $10K conversion - $8K resale tops = $6K loss).
This is why it makes sense to spend a little more upfront and find a boat with a good inboard diesel engine. I just also bought a new boat (a 1986 Pearson 28-2, little in common with the 28-1 that you are considering) and walked away from a couple of boats that appeared to need a repower. My 28-2 came with a pretty well maintained Yanmar 2GM20F.
An Islander 30 was one of our top choices too (the other contender was a C&C 30). One aspect that I didn't like was the fabric headliner that covered all of the deck hardware bolts. It made it difficult to inspect for leaks and the zippers to the headliners were frozen. In comparison my old Catalina 25 (a good first boat with none of the inboard headaches) had no headliner at all, and the Pearson that we bought has a rigid one with access panels to all deck bolts. I really prefer either of those options.
Electric inboard reduces your resale value because it limits how the boat can be used. It is great for a boat that is only going to be used on the bay, but the limited range makes it much less useful for short duration cruising (weeks not months or years). You can add a gas generator, but that's another big thing to be carrying around, and not something that I want in my cockpit when I'm forced to be motoring.
Anyway, I think you should be looking at other boats. It doesn't sound like any of these three are ideal for you.
I will say that a little smaller boat with an outboard (25' is about the upper limit) is a great way to get into sailing. Outboards are cheap and work well on smaller boats. Our Catalina 25 was motored in 4' seas without the prop coming out of the water using a 25" shaft length outboard. I learned a ton on that boat through lots of races, cruising (including a 12 day trip), and tons of day sailing. The smaller size (it's about 60% of the weight of my Pearson) made it more comfortable to single hand and the simpler systems made it very easy to work on. The are 22-25 foot boats available that are well suited to many types of sailing and that are a real blast. Outboards aren't awesome (they are noisy and even 4 stroke ones don't have great fuel economy), but they are good first boat motors.
You probably will get 3-to-5-foot-itis later on (as I did) and move up, but when you do the cheaper boat will be easy to sell and you'll have a much better idea of what you want on the bigger boat.