I recently discovered MDO. It's plywood in the middle with a thin layer of MDF on the outsides. Great stuff, half the weight of MDF and it edges and finishes just like MDF. Nice stuff.
MDO is the jam. Occasionally the Home depot by my work has it but usually I have to order directly from the Wood Mill guys.
As others have said here, use Plywood and if you can spare an extra 10-20$ per sheet, use sanded ply. It will make your life much easier.
Thickness wise you could probably get away with 12mm (~1/2") but make sure you have support braces inside. 1/2" is not as sturdy obviously so a couple of 1x6/2x6/2x4's on the inside for braces should be plenty enough to bear the load.