I would NOT go with Nema 34's - they are big, slow and need a serious setup of power supply and drives (high voltage/high power) to make them dance. If i were calculating I needed a Nema34 I would switch to AC servo's without further thought, a small AC servo can out perform a Nema34 easily.
IIRC the average "good" setting for micro steps is 1000 but it can be anywhere you want really, just remember that the higher you go, the faster Mach3 has to generate pulses - theres a limit (if you are using M3 to generate pulses that is)
Definitely belts as mentioned, another good profile is HTD5, very common and no backlash, its the shape of the teeth that matter, anti backlash belts generally have round profile teeth whereas the old ones had trapezoidal shapes.
Getting the balance between motor power and machine build is pretty much impossible first time - the calculations are horrendous so most builders think they need oodles of power and look to Nema34's etc - this then opens up a world of pain because you now have speed issues and your machine, although it can easily break a 25mm tool off, cannot move very fast when needed
On most machine builds, It seems the most common solution now is for Nema23/4Nm on something like a 70v power supply with good anti-resonance digital drives. I have these on my plasma table, and even though I'm only running 48v it can certainly shift when needed, One on X and two on Y, Z has a smaller 3Nm motor. This is gear on rack drive so not good for milling etc but adequate for plasma which has zero axis load.