answering to a number of points above:
That 0.9 deg motor looks to me be be a good contender.

There's bags of force with that motor, but my mind has now jumped to heating issues. Under normal conditions, the RA motor will constantly be moving (and Alt-z mounts will have both going 100%), and if you run these motors at full wack, then i expect will get very warm fairly quickly. So do you maths and check the torque requirements for your set-up.

Mitigations to this effect:
1. Most decent controller boards (phigets included) do reduce current to c50% the motors when in idle for a period (second or so), but as explained above, this won't help if the motor is on a long imaging run and side-real tracking!
2. You could run the whole system at lower power - BUT NOTE - by "lower power" means managing current limits to these motors (just lowering power supplies voltages won't have the desired effect!), if you need further explanation on this then I'll provide a reference and more details separately.
3. There are motor controllers that have RS232 interfaces that allow the controller to be programmed dynamically, (rather than the DIP switches). Now - I haven't investigated this so you will need to research - but it occurred to me that programming the current limits and micro-stepping values dynamically via software will open up a very useful way of maximising high speed slews on full step, whilst reprogramming to track objects using micro-stepping. By also cleverly adjusting the current limits (high of accelarations and lower power for simple tracking), you will also help reduce the temperatures of these motors.

Final point: don't under estimate the heating effects and hitting a temperature problem. These motors at full power typically consume 12v * 3amp = 36W or more. 1kg of steel warms up at the rate of 500J/C, so every minute at full power will raise the temperature by about 4Degs! (assuming no heating losses to air). Motors typically have an operating limit of around 80Degs before contact between internal parts might begin (the stator teeth start to rub and begin to make iron filings!), and bearing grease starts to leave by the nearest exit! - well ok, i'm being a little dramatic, but take heed, it will be an issue to manager you power settings on (not so) long imaging runs.