Prusa i3 Mega - Heatbed and extruder question

Hi everyone, i am building my first 3d printer and have decided to do the prusa i3 but the derivation with the 400mm x 400mm print bed.
My first question is, i plan on using a silicon heat bed and am trying to determine what to get, i want to do 4, 200 x 200, silicon heatbeds but am not sure what power supply would work, i was thinking about a 1000 watt pc power supply to act seperately from a smaller power supply for the motors, but was wondering if there is better option. Or am i thinking about this wrong and should use a 400 x 400mm heatbed, and if so whe re can i find one.
The second is that i want to put a dual extruder on this printer, but was wondering what everyone would recommend for the dual extruder on prusa i3.
Thanks in advance i really appreciate the help.

For that size heatbed the normal advice is to use a mains powered silicone heater. There are suppliers on Aliexpress that can make custom sizes and powers, although it's quite likely you can find an off-the-shelf heater for 400x400. You'll need an SSR to control the mains power from your control board. You could put together 4 smaller heaters, but it's probably a lot more straightforward to just get one the right size.

What are you going to use for the linear parts of the X and Y axis? I put a dual extruder on a normal 200mm X axis using 8mm rods and the amount of flex is surprising (mostly surprising that it can print at all). For a 400mm span you're going to need something much more rigid to get acceptable results. There's good argument for keeping as much weight as possible off the x-axis, so you could go with a bowden setup, or one of the flex drive options.

As far as the heatbed goes, I have a 400x400mm with I believe 400 or 450W of power and a dedicated PSU just for it. Wired through a DC-DC SSR. The 1000W psu won't be cheap if you're going that route.