Outboard for 1970 V-20 Cuddy

Hey everyone,

I recently picked up a 1970 V-20 for $500 bucks...The boat's in rough shape, but it had a decent trailer so I figured it was worth it. So far I've cleaned it up, put a nice dark blue paint job on it, installed a couple of new TODD seats, and done up the brightwork.
Anyways, the boat didn't come with a motor, which is what my question pertains to. I'm a broke college student and don't care about amazing speed, so I want to go with the least expensive option.

How much hp will I need to cruise at 17-20 knts.???

Also, do I need a 15 or 20" shaft???


Thanks for your help guys
-Eric