Welcome Steve to the forum, glad you joined us
1.) To get motor's direct from china you have to order several thousand, I have not found anywhere in China that you can order one or just a few.
The biggest difference in the motor's is the size ( CC's ) They may look the same, but thier is also internal difference's in some kit's. I think some dealers fire up the motor's and test them out before shipping to customer, and other's just box and ship out. But don't know for sure, just a opinion. And yes the price does reflect the dealer in most case's. I will say this, stay away from Revolution.com, very, very, very bad reputation.
2.) Don't know
3.) Your trek should work just fine but not positive since I am unfamiler with that model. Look here to get the demesion's http://motorbicycling.com/f3/what-ki...mmend-510.html
I have had no trouble with stress, I only know a few with crack frame's but as far as I know no need to do anything except good maintanence. Go over your bike, pack your wheel bearing's check your spoke's, ect. just good maintanece.
4.) I built a full suspension bike just for the trail, did not do what I wanted to so I sold it. Thier are a few here with some awesome full suspension bike's. Weather they use them off-road, could not tell you.
5.) I'm going on year five and well over 4,000 mile's. I have a friend in Nevada with over 7,500 mile's. I think about the average with good maintanece is around 5,000 miles.
I hope I answerd your question's well enough, other members should jump in and give thier opinion.