I converted my drums of my focus 1.8 to discs about 6 months ago.
I brought a full st170 rear axle, with brake discs calipers etc fitted from ebay from a scrapped car for £200.
Than i just had my old rear axle swapped with the st170 one at my local garage.
I had a look around first, and this was the cheapest and easiest way i could find.
hope this helps