Well, first, you should not need any motor mount, the DS I had a "kicker-pit" molded into the aft deck (it loocks like a small cooler to some!) allowing one to simply clamp the outboard right onto the transom. I'l linclude a pic below.
As to size, I use a 4hp Johnson (long-shaft) on my DS II and it works fine. 2-3hp is about right, over 5hp is too much usually, I bought the 4 due to the features that I wanted not being offered on smaller motors at the time (F/N shift, long-shaft, remote tank option, vibration damping). A long-shaft (20") is recommended, but a short-shaft will usually work, however the prop may be lifted close to the water's surface or out in waves (or large wakes from piowerboats). I did use a short-shaft (15") for the first 4 years that I had my Day Sailer, even on Buzzards Bay (near Cape Cod) it worked OK, but the long-shaft is a lot better!
Some owners use an electric outboard and those can work out OK as long as you have a way to keep the battery charged between uses and don't mind carrying the weight of a Deep-Cycle 12V battery (often 50-60# +). One great option (except for cost!!) is the new TORQUEEDO electric outboards, they incorporate a Lithium-Manganese high-performance battery, weigh around 27# all up (Including the battery!) and supply about 2 hours of power per charge. The downside is their $1600.00+ price!
