well I don't see why you'd use two psu's. why not just buy a better single one to start with ?
You can get a 650 watt for less than the price of 2 450s.
Why use 4 heat pads ? is it a size limitation ?

Rod wise, go for 10mm, if it's bowden and 12 if it's direct drive.

The only issue with z height I guess would be if it were a bowden setup. That's a lot of plastic to push through a tube, the flex and friction alone would make precision extrusion tricky. Direct drive, can't see any drive issues.

Don't understand why you'd need four motors or long belt thing if you have the build plate supported on all four corners.
As long as the bearing are good - you could probably get away with a single motor, and two height screws with the belt driving the two screws.
That would give you serious support and even travel over the whole bed. So four support columns at each corner and two screws center of the plate at opposing ends. Should be fairly easy to do.