Since it is a small machine you could use 5 TPI screws, so in full steps your resolution will be 0.001" per full step, and (like Ger21 said) you should not expect microstepping to yeild perfect positioning per microstep you should get pretty results to within at least quarter steps, which would be 0.00025" per quarter step.
If you are serious about accuracy I would forget the acme screws and find some reasonable priced rolled ballscrews. You can still use a delrin leadnut if you don't want the expense of the ballnut to match the screw. I used NSK brand (a top quality Japanese brand) of rolled ballscrew, about $150 for a metre of screw length. Even though classed as "low precision" ballscrews they are made by a good company on very nice equipment and turned out very accurate. If you buy ebay (ie Chinese) rolled screws the accuracy may not be near as good but they will be cheaper.
Backlash and machine slop, flex etc will likely be MUCH more than your desired 0.0005", as much as 10 times more as 0.005" backlash and slop are not uncommon unless lots of attention is paid to removing them in the machine design.