I would suggest buying precision leadscrews instead of acme screws. You can get them from Kerk Motion, and they are really not that expensive. If you don't have good screws you have no chance of having a solid machine. If you are carving and doing 3D contouring then you will want a good screw with the least backlash possible or you will not have a consistent surface. The Kerk screws are much cheaper than ballscrews, and they are just as accurate as a standard ballscrew. You may also be able to find the profiled lead screws in MSC but they are probably more expensive. Kerk has this teflon coating that makes the dust wipe right off too.
To decide on what lead to get you need to decide what you want your minimum move to be. Take the lead of the screw and divide by how many steps per rev that your motors have and see if that is good or not. If you are worried about whipping, then you need to calculate the critical speed of the screw and stay below that figure. I doubt you will be spinning them that fast anyway. For the 36-40" long axis I would recommend the 3/4 or larger screw. The extra expense is worth it in rigidity.