Concerning question #3, The cheap Chinese engines are the least reliable and require more maintenance that most other kits out there. If you are willing to keep up on the maintenance required, and work out the occasional problems they get, they can be the cheapest route to take.
The other kits out there cost up to 3x as much to initially purchase, but they require comparatively little maintenance and certain engines are extremely reliable.
My current suggestion would be a GEBE with a 4 stroke engine; EHO35 Robin Subaru.
Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More