What steppers?

Hi all,
I'm at the beginning of building a new small 'mill' and I'm on my way to
find some steppers. The problem is I don't really know how much torque I
should aim for. There is the 210 oz-in at Camtronics that looks good to me
but I don't know if it will be enough.

The base will be buildt from BOSCH's and ITEM's heavy-duty aluminum
extrusions, concrete filled if necesary. It will use linear ways and
ball-bearing slides (2 per axis). The leadscrews will be 16mm ballscrews
with 4mm pitch and the table will be 180x500mm (~7"x20") T-Sloted aluminum
plate 20mm thick.
My aim is to be able to cut holes in and engrave aluminium 2-3mm thick and
cut plastics etc. But it would be nice to able to do some 'real' milling in
aluminum. The motors will be direct coupled to the leadscrews.

So, my qustions to you guys is: What steppers should you choose if you were
building a machine like the above? Right now I have drivers capable of 2A @
40V (L297/298) but if heavier is necesary I will just have pay the money get
some better drives. It would be good though to use the 2A drives.
Dan Mauch says that the motors (210oz in) will have plenty of power if used
with a 20tpi screw but I'm just not sure how it will perform with 4mm pitch
ballscrew.

Thanks for your input!
/Henrik Olsson. Sweden.