What exactly do you mean by "kit car"?
You should look up OBDII dataloggers, if your car supports that. They are very easy to hook up and can give you quite a wealth of information for environmental awareness without the need for additional sensors. This also has a lot of Engine Monitoring, so these things can be moved up to phase 2 or 3. Some of your ideas however seem a little, well, not very useful for the amount of work that you'll need to put into it.
What's your budget for all of this? Maybe then we can offer some you some pointers.
You will need a regulator if you want to use that case and power supply that you mentioned, however if you did a search on this forum you would already know that (hint hint)
A lot of the information that you are looking for is already covered in depth in this board, just try to use the search feature to figure it out. My FAQ might also help you, located here: http://www.mymp3car.com/MP3Car/mp3carfaq.asp