I have a 1650 Lowe Roughneck w/ a 20hp Merc. Its alright. I went with the 20 because a local lake has a 20hp limit. We dont have huge lakes where I'm from so its not a big deal. With one passenger and my dog and some gear I get up on a plane just fine. However, add one more person and its slow going. If I lived near a place with big lakes I would go bigger. When I'm fishing with just one other person I can cruise around the lake at a pretty decent speed.
Honestly, if I was you and I didnt have a reason I needed only a 20hp I would wait and save the extra money to get a bigger motor, but if you ABSOLUTELY have to have a 20hp and cant wait you will be okay.
Nobody ever complains about having too much motor power.
BTW the 20hp merc runs great, turn the choke on- starts right up, even in single digit temps.