Use Linux
1) Samba
2) quotad
3) ipmasq
There are plain-english How-To's for each of these things available at http://sunsite.unc.edu .
May not be as "pretty" as win2k, but you will have better performance at lower cost (about $2, or your own blank CD).
If you choose NT though, go with either NT server, or Win2k Server. Pro doesn't have quota support.