If you really want to build a rally car then go for it but be aware that it's not the cheapest way in to a rally car! I'd also be wary of getting tunnel vision as far as ford is concerned. The CVH is an OK engine but was never really developed much in terms of rallying. I hate to say it but a 1600 Vauxhall engine would be a better bet, vauxhall engines have been developed far more over the years as the Nova has always been a popular rally car. The nova would also work out cheaper to build than a Fiesta as the parts are readily available of the shelf and there's tons on the S/H market. If you want cheap rallying then go for something like a 205 cup car or something that isn't maybe as desireable such as a Sunbeam or Avenger, you can pick up a second hand Sunbeam or Avenger out of the Motorsport news for less than £2K a reasonable 205 cup car will cost around £3K. The other thing you have to consider is how easy it will be to get for spares as yo will break lots of things, particularly, TCA's, struts, springs, steering racks and no doubt there will at least be some panel beating/replacement panels!
Whatever you do all the best and keep us posted on what you doin!
Here's a couple of links that might help:
www.raldes.co.uk