I currently use an MDF router to the free JGRO design - there's plenty of info around about it. I built it a couple of years back, and have used it a fair bit. MDF is probably not as stable as ply, to be fair, but it has a number of limitations. It isn't very strong, which really restricts cutting speeds which are painfully slow. In an unheated garage, it isn't very stable which means that particularly bed alignment varies so depth of cut is difficult to maintain across the bed. It is difficult to maintain bearing alignment, so you are continually adjusting. I use threaded rod as leadscrews which gives slow speeds due to whip. You could fit decent bearing rails although bed distortion would make those difficult to keep aligned; ballscrews would give better speeds but the bed structure couldn't take the cutting loads. In any case, by the time you have bought decent bearings/ballscrews, the cost of the structure is a small part of the total. It's been great as a starter project where all but motors and electronics are considered throwaway but does the fact that I'm building a steel router now tell you anything?
Rather than look to the US, take a look at cnc4you, Zapp Automation, or other UK sources if you don't want to go to China.