You might be on to something with the VTOL concept, make a quad-copter attachable under carriage for the plane, attach, take it up, apply plane thrust, fly around, bring it in, apply quad thrust take into hoover, bring it down. epic.
you might want to consider the size of the quadcopter.
If you are going to make a heavy lifter for such a weight a small Quad might not be the best way to go because you´ll need a nice low kv outrunner and also some very BIG propellers. If the motors are too close together you won´t be able to mount props with the sizes needed.