In my opinion, upgrading the TM MP7 isn't that difficult.
If you can handle modifying a TM M4A1, then you can do the MP7.
As always, if you have NEVER upgraded any AEG type before, I would highly recommend AGAINST anyone trying to upgrade their MP7 or AEG...
Jugglez: In regards to the difficulty in putting the gearbox back into the body, you are probably talking about getting the wires back through the opening on the side of the body right???
Solution: Get a paper clip, or a wire, unravel the paper clip so that it's kind of straight. Stick one end of the paper clip into the + or - wire and wrap it in some electrical tape. Thread the paper clip wire through. Easy peasy