My starter machine for milling PCBs used TR10x2 screws and those motors with 30v drivers. It was all made from MDF and the overall work area was 500 x 300 x 150 but I actually rarely cut a PCB bigger than 140 x 100 and even that took a while. The limiting factor on PCB size/complexity was how long I could run the spindle (a modded DeWalt die grinder) before terminal deafness set in on anyone within 100m radius.. Think about your spindle (Kress or similar?) and where you will run it and for how long (a 6hr run isnt unlikely) - that gives you a clue to the speeds you need - and what work area you could accomodate within some sort of soundproofing environment....