I don't have a motor to rent so the rest of my reply might be a waste of info to you but here is my two cents. Watch craigslist and pick up an inexpensive unit. I would go with the smallest HP that you can muster. Watch the video on the first page of this thread.
3.5hp motor sufficient?
That was a 3.5hp motor pushing 11 rafts. Your not going to be able to accomplish big speed with a motor, just enough to help from the rowing if you need it. So go with a small one that is light, creates less emissions, and you carry less fuel.
I prefer no motor, but I think they have their place. Especially when you just don't have the days to do more miles. But beware many folks HATE motors and will "shun" you. A buddy of mine floated Deso last year with a motored group and when they passed other groups, some of them would all turn their backs with no words or gestures, ust "shun". I laughed when he told me that story, but really how sad that there are some people that cant understand that some people just don't have the days to do it like others.