JH313,
I'm glad you're considering joining the Ultrasport club. You will not be dissappointed..........
If you are still trying to decide which size, I'd suggest the 60 size. I have also seen them fly with both 4 strokes and 2 stroke motors. The plane flies well with either one, but the vast majority I've seen have been using 2 strokes with the OS 75AX being the most popular. Your choice of a 91 4 stroke will be great in the 60 size. One great thing about the US kits, the instruction give different locations for the fire wall based on whether you use a 2 or 4 stroke motor.
If I may suggest, I'd recommend building the plane as a tail dragger and buy a set of the E-Flite electric retracts. They are more expensive, but there is no hassle with air or linkages to worry about.
David