As regards the pots - if you have 500k in them, you can solder a 470k resistor (can be 1/4 watt, there is not much this resistor needs to dissipate) from the tap on the 5-way-switch to which the coilsplit connection (that would be the green and white wires soldered together) is connected, and the output lug on the switch. With this set up you will have a 500k volume pot when the humbucker is selected, and a close to 250k pot when any single coil combination is selected.
As regards pickups - I still think you should go for a ceramic, so Miracle Man or C-Bomb.
Cheers Stephan