They've changed the pedal sensor now, so it fits more bikes. You have to drill the magnet disc if you have a Octalink or Hollowtech type BB. The only other thing is the brake cut-offs. They provide new brake levers that only work on cable brakes. It's a good idea to add a Cycleeze torque arm, so you need a bike with a suitable anchor point for it like a disc brake. Rim brakes aren't really suitable for electric mountain bikes anyway. Lastly, you need one with a triangle big enough for the battery.
The ideal donor bike therefore has a largish triangle and disc brakes. Combined changers and brakes are not ideal. Nothing else matters really.
You get about 22 mph in a 26" wheel and about 25 mph in a 700c, but you're trading speed for torque. If you're heavy or have steep hills, 26" would be better. The 700c rim is not vey wide, so avoid bikes with big typres like 2.3" wide.
I recently fitted one to a Boardman hybrid, which was on the Gadget Show 27th April. It's a very fast bike, and would be ideal for high-speed commuting. It can average 22 mph without pedalling too hard. The guy that has it now (Saneagle) rides it without power most of the time and can still keep up with me. He weighs over 110kg! What it shows is that a large hub-motor can still work in a light bike. The compete system of kit and bike makes a very fast and efficient means of transport.