Hi Ben and Welcome.

Plan is kind of OK in some ways but wrong in others.?

Not bad plan regards buying Electrics which will move with you but NOT the MDX-340.
PMDX make great products but this is limited to some degree because can only use 3A motors and run them with 36V which doesn't give much scope for upgrade and limits to small motors. Simply not enough volts or amps for larger motors, even nema23.
Most of the other things it provides are just bells n whistles you don't need.
You'll build a Much better system for Not much more money which uses 50Vdc Digital drives that will blow the PMDX drives away.

Regards re-using ballscrews, rails then this is were plan goes wrong. You'll be much better just building the machine the best you can and then sell it when finished.
Most mills will use different pitch screws and methods of fixing etc so most likely not usable of more trouble than worth.

Personaly I'd say wait save up and don't build anything rather than use this plan because large chance of costing money.
Better option is to build a machine with half decent everything suited to that particular machine and then sell everything on when ready to move on. You'll get more for working machine than the Sum of it's parts provided it's not made from MDF and Threaded Rod etc..
This way you'll get exactly the right stuff or machine when you move on and often the case the existing machine will pay for it or go big way towards new machine/parts.

Good Luck and Don't go rushing to buy anything it's sure fire way to blow money.!!