Rear camber is the main issue. You can get camber bolts, but corrosion can prevent them being fitted as per what happened to me. If that happens, then you can drop it as much as I did, but any lower would cause issues.
Anyway, I've now got a VW Passat due to starting a new job and the Focus is up for sale. I got the mk3 Focus 18" wheels refurbed in shadow chrome and put them on our candy red Mondeo. They look pretty awesome!
So if you're interested in the Focus, send me a PM. Looking for £3.5k. Car has only done 50k miles.