so you've either got an 89 spec dohc or a 92 spec gt yes?

seeing as everything else came with discs all round in the dohc flavour AND the larger servo and master cylinder

so what you need is the following:
dohc all disc servo/master cylinder combo
mondeo mk3 calipers
st170 focus 300mm discs
disc'd rear end from a sierra, any sierra
16 inch rims

with a bit of haggling and luck, you shoudl get this lot for around the £400 mark

when you've got this lot you can fit them together, without this lot you are going to have brake balance issues as well as soft pedal travel issues because the calipers are too big for the master cylinder you've got to psuh fluid into