Nice looking machine. It's similar to what I'm working on as well, but my gantry will be a bit different. But the 1kW servos (although mine are ebay specials), rack and pinion drive and dual spindles are similar although I'm going to do dual screws on my z axes and have one router and one milling / drill head and sadly no ATC's (maybe in the future).
Have you worked out any details of your rack and pinion setup yet and how you'll eliminate backlash in it? From your drawing you have a helical rack, but any details other than that?
Do your concrete bases just sit on the floor or are they bonded or bolted to it? If your frame weighs 4500 pounds, you probably won't have issues, but I'm planning on something a bit lighter and am worried about the machine walking itself around over the floor. I'm calculating I'd need a minimum coefficient of friction between the machine and the floor of 0.5 to prevent it from walking around under max accel or decel. I'm just wondering how you approached this.