I'd suggest you go to the CAMS manual and read the 'Tarmac Rally Technical Regulations - Modern' as even if you do not ant to build the car for tarmac rally the person who buys it off you may.

A ferw things to consider based on my recent experience:
1. How much money is in the budget to build the car? Add 50% to whatever you think it will cost
2. Do you have the skills and more importantly the desire to continue working through the project when it gets hard? If it wasn't for a few mates my evo would still look like a shell. But it is satisfying when it is finished (almost)
3. What do you want to do with it?
4. As someone said obtaining B reg is very difficult at the moment, I understand that even getting long term permits is getting harder.
5. When refering to tarmac rallying the base car you start with impacts the competition weight. So a Spec C MAY be a better coice that an S202 as your race weight will be lower.

Hope that helps.