You are looking at a couple of issues.
One, quadcycles may not be road legal without being tagged, and insured as an automobile--and you may have to have a driver's license to use it on the road.
If you are planning on putting two motors on the bike (one for each rear wheel) synchronizing them may not be as trivial as you hope. If you only go with one you still have a single wheel drive bike. Two motors also doubles the cost, and two sets of batteries doubles the weight.
The trykit is for British racing bikes and is hand built, it's not going to be inexpensive. Also the trykit shows disk brakes, they are not commonly found on hub motors. It may actually be cheeper to have a custom frame built than modify an existing frame.
Personally I would recommend keeping it simple and just add a rear hub motor to your current frame. If it ends up being inadequate you already have half of the parts you need.
If you are dead set on making a quad, I really would recommend going with a mid mount motor like the stoke monkey (clever cycles makes them). A quad is conspicuous enough. A stoke monkey really isn't going to draw any further attention.