Hi there
I just came up with an idea for a cheap and strong servo: Why not use a windscreen-wiper motor from a car?
Years ago, I once bought such a motor from a scrapyard, and I think it is strong enough to power the heaviest mills seen here. That is because these motors (well mine anyway) have a built-in reduction gear, thus reducing the rpm to about 50.
You'd have to combine this with an encoder, but I thought it should be possible to make that yourself, using a CD-sized transparent disc (you can find these on top and bottom of spindles with 100 CD's) and a transparent label with stripes on the side. Then all you'd need is a led and a phototransistor or something like that, to finish your encoder.
A little math: A CD has 6cm radius, so the edge is 2 * pi * 6 = 37.7cm long. If you want 200 stripes, they would be 37.7 / 400 (don't forget the spaces between them) = 0.94mm wide. This should be easily detectable for your sensor, and by using both the positive and the negative edge of your sensor's signal, you'd have a 400 pulses/rotatation encoder. Not bad for something homemade.
What do u guys think about these ideas?