People don't often post expenses because they've heard - or already know - how costly divorce can be
Regarding the Seafari, there were plenty of early outboards made. Bushwacker's and mine were both '72s... I think the mix was pretty even except in '71 and '72, see chart:
http://classicseacraft.com/brochures...production.pdf
If you want an OB get an OB. You can find a decent old merc or evinrude 115 for around a grand all day long in FL.
I think you could easily find a decent running Seafari in the 3k range. If you did the transom yourself you could probably do it around $600 of materials.
If you found just a neglected hull you could likely get it for under a grand on an old trailer.
Quality flotation brackets ain't cheap, probably 2k or better; maybe as much as neglected running Seafari itself. If you go for a bracket make sure it is a flotation bracket as you'll be moving the center of gravity way back, and that will prove significant. For your target price I'd stick to older 2 stroke motors in the 115 hp range that will weigh in @ 290 to 330 lbs and hang them on the transom. That's kind of the target motor weight on the older 20s. That being said I would suggest you find one with a 25" transom or plan on raising it. I'd probably ditch the bracket idea unless I planned on repowering with a 400 lb + late model motor.
The 1970s 20' hulls are pretty weight sensitive because they have a narrow beam 7'6" and vertical thru hull scuppers that let water in if they have too much weight aft. Of course if you're diving you'll get wet feet anyway.
Good luck in your search. I personally prefer cuddy cabins for the camping element and shelter from the storm but ccs are great dive boats too.
As far as budget goes, if you have a target in mind... plan on doubling it before you're happy