ROB......First off, expect NO income and help from sponsorship apart from possibly discounted prices. THey will not risk their money on a low exposure unknown quantity driver , especially of your age with no experience and results. SO basically your on your own.
Buy a cheap car, to get your cup up so spec, even though Grp N is showroom spec it will still cost loads.
You will need a minimum of 6 sets of wheels and tires with diff compounds and treads, FIA homologated cage, interior stays as it is, you cant remove anything, you have to keep the std glass, no plexiglass, std suspension pickup points, std suspension arrangement (and the availability for clio suspension parts that are any good is not good). Um, the box is free as long as it remains the H gate shift pattern, no dog leg boxes.....um..
You can seam weld, cut half the loom out prep would need ot removing everything from unused connectors and grommets, fire prrofing, foam fuel cell.
Thats at least another 10-15k on top of your cup price.
I cant remember/dont know a shed load more, but i would of thought you would of known the basics....