by cvoinescu » Mon Aug 26, 2013 12:30 pm
I won't touch the first part of your question, except to say that most 4-wire bipolar NEMA17 40 mm motors with a current rating of about 0.7 A to 1.7 A will do.
For the second part, because the Z axis is screw-driven, you have more resolution than you need. To get extra-fine layers, in addition to a decent Z resolution and positioning accuracy, which you have, you need a small nozzle. However, the smaller the nozzle is, the harder it is to push the filament out, and the smaller volume you can extrude at a given time; the difficulty increases very sharply with the decrease in nozzle diameter. Nozzles below 0.3 mm or so are impractical. Other people have experimented with ultra-fine layers, and the consensus seems to be that about 0.2 mm is the thinnest layer that still gives good results. In the test I've seen, 0.1 mm looked worse (and took much more time to print).
Skeinforge has an interesting feature, where it can divide layers into a number of thinner layers only on the outside of the part (the perimeter of each layer). With that feature enabled, the outside surface of the object is printed at an increased resolution, but the bulk of the object is printed at normal resolution, so the time penalty isn't that great.
To sum it up, don't worry about the Z resolution, it's better than you can make use of.