i have run 4s in my 9 lb truggies and it works but things are much better on 5s now. so i would at least go 5s in your setup. also, a 1515 would handle the job but i think a 1521 would be more ideally suited. it will fit just fine. it is the same width as 1515 only about a 1/2' longer. also the mounting screw holes are standard as on all electric motors. if you search google you can find sites to buy 1521 Neus. lots of times you can find used ones in different forums too. Lutach got a 1521 last week for $70. i was pissed. he beat me to it by like 3 days. anyway, if you have trouble finding any pm me and i can give you some links that i have. oh, check the boat rc scene. 1521 Neus are pretty much standard equipment on rc racing boats. so often you find used ones for sale on the boat forums. good luck. oh, and i would highly recommmend the Neu over the Medusa. I havent tried the Medusa but i can only go off what i hear from those who have both. Medusa's are good motors it sounds like but the Neus just out perform everything else out there.
check out this link of a (i estimate about 10-11 lb) muggy on a 1521. does standing back flips.
http://www.youtube.com/watch?v=bo7GdkNpKAY