If your overloading a z4000 already, I think an automobile battery charger might be the easiest cheap way to get the amperage. But thats DC volts so you might need to reorganise some things for issolation (and some items need ac) The G crowd makes good use of them regularly and I don't recall any DCC issues being mentioned, so hopefully the one you choose is "clean enough" for being around ANY command system..?
If you can find a used Astron or similar ham radio power supply they ought to be great. The last one I had was 12v dc 50amp (13.7v no load). Only 5a less than my roll out Snap on battery charger switched to "start". Very steady, very clean...very heavy, lol 
Stans pwm boards and/or a vintage lionel rheostat for the variable. A new rheostat isn't cheap. One thing a rheostat does offer over the boards is the ability to use one control with more items on the circuit. Since the board has a 5a max. you'd need a bunch of them.
Most modern dimmers need full household voltage. An
