Foamboard in europe usually comes in sze A4, A3, 500X700mm and 1000X700.
If you want to fit all of the wing halves onto one sheet you'll have to order the biggest size, though splitting the wings shouldn't be too much of a problem, just put the joint where the flaps end and the ailerons start.
Same goes for the fuselage, though you kind of have your split already set up for you where the tail starts to taper.
European foamboard usually comes in 3 or 5mm thickness, with 5mm being the way to go in my opinion. it's a little thicker than what Flite Test use for their speedbuild kits, but if you cut your parts as per plan everything will still fit together nicely.
Lastly, make sure you get foamboard with a smooth, extruded polystyrene core. It's both lighter and easier to cut than a PU foam core.
Be prepared to work a little harder getting the paper off the edges and fold channels. I don't think the storch has any large areas needing to be de-papered, but carefully using a sponge to soak those areas helps.
