Milling steel, ali and brass no problem as long as you take it steady.
Definitely get the belt drive kit, its well made and easy to install - not worth making one yourself.
I converted my mill over to CNC and its the best thing I did, have look here:
http://www.cnczone.com/gallery/showg...page/12/sort/2
I used L297/L298 chips as the basis for the driver cards, built an opto isolation card to decoupled the PC from the power side of the drivers, found some motors on eBay and linked the X and Y axis up directly. Z axis is driven with a ball screw and purpose built mounting plate bolted to the column.
Software is EMC/Ubuntu Linux running on a scrap PC - works really well and the EMC forum is really helpful and friendly.