I'd go with the 18 hp
I replaced my old Atomic 4 last year with the Yanmar 2GM20F 18 hp model. The power in that motor is more than adequate in most conditions, but even with 18 horses I sometimes wish I had a little more power. I know Catalina still makes the C-30 and they install a 3 cylinder diesel (~25 hp I believe). When I replaced my motor last year the 3GM30F motor was an additional $1,000 and I didn't want to spend the extra cash. Now that the motor's in, I wish I did. I figure the life of the motor is going to be at least 15 years...if you spread the cost of the bigger motor out, it's only $67 per year more. If you're going through the expense & hassle of a repower, I'd get the largest motor that your budget will allow. Just my 2 cents.Allen Schweitzers/v Drambuie, hull #632