on the rear you will be better off fitting the whole rear beam as the holes in some of the drum beams aren't big enough for the disc stubs to go through

if you are going to do whole abs change though make sure you get all the correct bits from the same car as there were 4 different sorts that spring to mind

the early set up had integral valve blocks while the later ones had seperate ones on the other side of the engine bay

then there are the versions with the resivours on opposite sides due to the exhausts being on the same side as the master cylinders

what would probably be best is to get the bis first and then find the larger dohc uk spec master cylinder and servo set up which will run the bigger discs at the front and the discs on the rear before you start wiring things up

as said before, the 4wd ecu and wiring is different to the 2wd version, the 2wd version is much better

abs front discs are 260mm and rear discs are 253mm

if you want to go to 2wd cosworth discs you will need specific cosworth 2wd hubs in order to bolt the calipers on the front and the discs on the rear as they run 283mm front discs with fixed 4 pot calipers and 273mm rear discs with a taller hub carrier
you acn also fit the 4wd vented rear discs to the same carrier but you will need the calipers to match the vented discs, along with the caliper carriers as the 2wd discs are 10mm thick and the 4wd discs are 20mm thick