Rust free MK 1s and MK 2s are pretty thin on the ground and are commanding high prices. 3 to 6 grand for a MK1..
The best advice is a tatty unmolested MK1 for £500, spend the £900 on the sills and you have a good base.
In terms of build cost £2k for the kit, £1500 for the repaired car, another grand for wheels, tyres, paint etc and you have a good starting point for £4.5k.
If i were doing it all over again I would buy something like this as a base
http://www.mx5nutz.com/forum/index.p...pic=364601&hl=