Thanks for the replies everyone.
Inferno115, the conversion is a bit of messing around and there are many ways of going about it. Cosworth engines are getting older now, so finding a good engine to start with is getting more difficult.
Being an RS owner you have much more space to fit an intercooler which is nice, There are many little bits and peices that require alot of thought.
If I was looking at doing another conversion I would use the original sump, which requires custom engine mounts and some trimming of the firewall lip to get the engine further back, I would also use the T5 gearbox which requires a larger gearbox tunnel. And an aftermarket ecu is something well worthwhile as well.. as the mine has never run as well as it should with the standard computer and there is a lot of messing about with old wiring looms etc.
Bearing in mind that you will also need to upgrade your brakes and diff on top of this I would say that $15k is a realistic price for such a conversion if you are smart. I wouldn't bother trying to cut any corners as it comes back and bites you (as I have learned the hard way over and over (uni budget, haha)) Make sure every single component you use is new or rebfurbished.. otherwise it just ends up costing you more in the long run.
So I guess do your research and do it once, there are many of us out there now that have done the conversion so there are a lot of people to offer you advice on how to go about it.
Cheers,
Damion.