Well done. Looks you got a great boat in very good condition dirt cheap.
The boat with floorboads probably weighs ~ 250 - 290 lbs. Chances are you will want to trailer that boat, rather than set it up and take it down with each use. In that case, an outboard like what Nos4r2 described (which would weigh ~170 -175 lbs) should be ideal. The extra horses will make a big difference in performance with very little effect on gas consumption compared to a 25 - 30 hp. See what the transom plate says about it's max hp. If it is rated for 50hp and you can find a short shaft tillered 50hp yamaha/mariner (same weight as a 40hp) or tohatsu/nissan (~155 lbs), that would be even better still.