For a recent discussion of trailers (both DIY and ready-made) see this thread in Living Car Free -forum:
http://www.bikeforums.net/showthread...hlight=trailer
As for building a motor-assisted trailer, methinks it's a bad idea. There's a whole bunch of safety issues to deal with (how to prevent jack-knifing, for example). And would it be street legal where you live?
--J