sounds like a 4 axis mill, no? I'm currently designing/building a cnc router. for cheep ball screws and linear bearings check out ebay. I just bought 3 ball screws and linear rail sets 500mm 1000mm and 1500mm travel for around 500 from linearmotionbearing2008. ball screws are precision grade c7 so you'll get .002" lead accuracy per foot and the backlash is typically around .0005".
You need to be concerned with the rigidity of your machine. from what Ive found you want the following minimum for milling:
50,000 lbs/in deflection for steel
12,000 lbs/in defection for Al
2,000 lbs/in for wood
this is between the cutting tool and the work piece.