The motor is usually disengaged by a centrifugal clutch, but I've seen some with Manual clutches
Yes, they are a bit hard to pedal. So you may want a different bike to bicycle on
And they are safe at speeds of 35 to 45 depending on brakes, suspension, tires and rims, and the frame. Mine tops out at 63mph, which is VERY unsafe, do I ever do that speed? Very very very very rarely, if at all. But at 35 it is safe if your careful.
If you have any specific questions, feel free to email me.
And why pay someone to build it? I built mine from things I had laying around, feel free to check it out. So the kits will be very easy. And if you get stuck on the kit, well, there's a forum for help xP I would encourage building it yourself! They are great fun to build! I built my first one,Chelsea, at 15, so it's do-able.