'79 CC with '85 merc blk max 200 hp

[Serenity]

Thinking of buying this boat mainly for hull. Hull,deck, etc in good original but good condition. Trailer needs work and motor has low hrs for age. Plan to install either single or twin 4 stroke o/b later. Any idea of value on present o/b? Any comments on going either single or twin. Usually fish between 20 to 30 miles offshore, half of time with buddies on other boats in area. I have 20'Sea Craft cc with I/O now but plan to sell it after I get new purchase set up to my liking. Appreciate any comments.