The sticker in your hand book or boot will have your gearbox code on it, 3 letters to the left of the paint code. Pretty much any, early mk4 1.8T box will fit, although some of the ratio's may differ slightly.
The Cupra box is a 5 speed so will most likely fit, the Cupra R is a 6 speed box so you would need flywheel, clutch, linkage and driveshafts from the Cupra R too
