1)It shouldn't be difficult to upgrade to the acme, you will just need to turn/have the ends turned down to utilize your bearings, and stepper coupler, and supply a new nut for each axis. Make sure to give yourself enough clearence from your linear assemblies for the larger acme.
3)for the frame, ultra precise is not necessary UNLESS locating your slides, and motors/leadscrews/bearings is dependent on high precision to line up properly. In other words, do yourself a favor and make things adjustable. Other than that, you just need to be certain that the axis' move 90 deg from eachother.
4)If using skateboard bearings (I never could understand why they are always called "rollerblade" bearings, but then I skateboarded before I ever rollerskated or rollerbladed)wrap som 22ga bare copper wire around the lead screw where it sits inside the bearing. Keep wrapping until the fit is snug. Or, use a nylock nut on both sides of the bearing, facing eachother. The side with the nylon lock is round and slightly tapered so that it sits nice in the inner race and centers the lead screw.
Get all of that? I am quite long winded sometimes, but I hope that helps. Good luck, and keep the posts comming!
Stop talking about it and do it already!!!!!
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)