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OMG yep, any sailboat that has an outboard is the end of the world!
Maybe we should ask where you are sailing first. Cause on a lake it wouldn't matter much.

Most of the other issues look like typical 40 year old boat problems.
Oday made pretty good boats, and yep many manufacturers skimped on things (in their defense the boat is 40 years old). The backing plates being replaced would be high on my list if it were mine. Unless I was sailing in big(ish) waters, the outboard wouldn't bother me.

I don't believe the Odays came with saildrives (maybe on very recent model years). Nearly all I have seen were traditional inboards, but his point is correct make sure the hull was sealed properly, around the prop shaft strut and such. The poor mans solution to that was leave the engine in place and just slap a motor mount on it and an outboard. That gives you all the risk of an inboard with none of the benefits.

You gotta determine if $4000 is a throwaway boat or not. Over time you can decide if any of this should be upgraded/fixed, focusing on those thru hulls first.
Just never look at the boat in such a way of worrying about whether you get your money back out of the boat.

 

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An outboard mount sized correctly for the boat should swing a pretty good distance along the transom.
While not ideal it'd be fine. Electric WOULD be ideal though.

Given all the caveats of this boat, title, sketchy outboard conversion, I'm less on board with it. Unless of course $4k is a throw away boat for you, then honestly none of this matters. I've bought some pretty sketchy boats and managed to make them work, and you could too. I prefer mine to be trailerable though so that I can haul them out myself if I need to do serious work. That's just me though.

My last project boat was a C&C 32, and it had "issues" but I still loved her. There are days I wish I still had her.

 

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You can put a trailer under any boat. That does not make it trailerable. With enough effort you too could trailer the Oday 28.

Generally speaking its weight, and width that put something into non-trailerable category. Given the width of the Oday 28, you are at 10.25feet wide, maximum width on roads (without permit) is 8.5 feet. I know, I also exceed that amount 9.25, but the other thing that will kind of make it untrailerable is weight. 7500 isn't horrible, but add a 2500lb trailer and well, you are pushing 10k, which by the way is very doable for a modern 2500 or 3500 pickup truck, but add the incline of a ramp, and slick ramp and things start to get sticky. Have to do a strap launch and it starts to get downright scary.

Final thing is a deck stepped mast (otherwise you need a crane). The tabernacle needs to allow some kind of gin pole or A-frame for lifting. Again you could crane up the mast, but at that point you could also pick the boat off the trailer with a traveler too, but that's not really the spirit of "trailerable" now is it?

Again ALL of these things can be overcome and you can technically trailer. Many of the things triad has available to make your 28 trailerable were developed with people who trailer, a friend was one, with his Catalina 27 tall rig. So it CAN be done, but doing it is not going to be easy for any part of the process.

While I see the 28 comes in a shoal/centerboard configuration that will help with launching (which is usually why you get a trailer - so you can launch it yourself). Mind you fin keels can be launched too, if the ramp is long enough, and steep enough, and YES I have done it, but you do it at these weight classes, and its not going to be pretty.

All the things above combined make it a difficult boat to trailer launch/retrieve (but agreed not impossible), but generally not considered "trailerable." For the record my boat would probably not meet the definition of trailerable either, yet I do. My boat does have some things that make some other parts of that process a lot easier though (no keel, simple pin in deck stepped mast tabernacle are examples).