Malcolm. Firstly, there is no build manual but if you search Google you will find several build blogs that detail the build. It is not a hard build and if you have some basic skills, you should be OK. As for Ford vs Mazda, that is personal taste. If you go the Ford route, you could buy pretty much everything new so you could get a new registration, but don't be naive about the cost, you may not see much change from at least £15K! If you go the Mazda route, you can use a lot of the donor vehicle and get an age related plate, although it will not be the same one as the donor car. The cost will be a lot less.
Hopefully others can add a bit more for you, but I would definitely recommend coming along to the National Kit Car Show at Stoneleigh at the start of May. You will get to see lots of cars and get chance to talk to the owners in more depth.