Like a lot of you (mxrob as well I'm sure) I got tired of hugging the bucket while holding onto a wimpy 3/8" drill that overheated before I could mill enough for a strong (or large) batch. Unlike mxrob, my solution is much more on the low end, employing mediocre carpentry skills and less than elegant (ghetto?) engineering.
For the motor I found a low-speed heavy-duty 1/2" drill at HF that just needed a way to apply latching speed control and a method of mounting to the cart. Not so pretty, but it works just fine:
I had a hard time finding a real motor (with gear box) to fit my budget, so I looked elsewhere. The drill does not have a trigger lock, which is kind of lame - even for $50 (I believe that Blk&Dkr make one in the same price range but I think it only locks at high speed). So as you can see the latching speed control is provided by a bar clamp. Actually quite functional and adjustable.
I figured once we're on the low road, let's keep the theme alive for the whole project. The drill is mounted into a cutout and positioned in a "saddle" provided by some pieces of vinyl baseboard screwed down to the cart top. I mounted my trusty Barley Crusher on it's original baseboard to shim it up; the drill and the mill mate up very well indeed.
I had thought of wiring up a nice switch box to take it up a notch, but for $7 the power strip does the job, and again - stays with the theme.
A couple of coats of poly, some medium duty casters and two blocks placed provide foolproof positioning of the bucket and there you have it.