just to reiterate what has already been said, if you can find a used Yamaha 130, that is perfect for the 18' (and I don't think finding one would be all that hard, I see a lot of them around).
Your biggest concern if you aren't going to raise the floor should be weight rather than horsepower. That's a tail-heavy boat and putting a heavy outboard back there won't help at all.
I saw an 18' just yesterday down at Birdsall - I still sort of wish I'd kept my 18'
