It also depends on your intentions for the motor. If you are only using it to get in and out of a harbor, 2hp/ton should work. But like 928F and dman have said, if you are looking to be able to make time while cruising to weather into a seaway, 4hp/ton is minimum. Add to that the loads from alternator/refer/etc and you may want 4.5 - 5/ton.
Prop pitch and sizing is CRITICAL in a repower situation. You need to take into account displ. waterline, gear reduction, etc. There are formulae to figure it out, but it is best left to the pro''s AND have second and third opinions done. Nothing beats a three blade fixed prop for motoring, but it will add drag while sailing.
On my 37'' 13,500lb sloop, I went from a 14hp 2cyl Volvo to a Yanmar 3GM30F 27hp. Once the proper prop was put on (in my case a 16" 2 blade Martec folding) the difference was staggering! I could now make time motoring into steep 6'' - 8'' waves and 20Kts+ winds. I would have been stuck in the harbor with the old set-up. I was no longer stopped dead with every wave hit.