Quite a few people have done Zetec RWD conversions. Rally design and Burton engineering do most of the items you will need.
I've looked into it but never gone further than an investigation but you will need to fit a spigot in the rear of the crankshaft to take the input shaft from the gearbox, re-locate the thermostat, get engine mounts made, acquire a rwd exhaust manifold. You will need to either use the Focus ECU or get an aftermarket one, and also consider if you want to use the Zetec injection system or go for twin 40DCOE weber carbs. Then there is starter motors and alternators to think about as well.
I assumed that I would buy a 'crate' engine for about £800 and the total cost would be in the order of £3000
I'd suggest you Google Zetec RWD and see what the turns up.