Ultima GTR (buy pre-built for ~£45k) then a custom 1khp Ultima Evo to build myself, not both simultaneously.
You can build a 1000hp Ultima GTR, that's just engine stuff, but the Ultima Evo has an improved chassis and some cosmetic / QoL upgrades.
C6 would be LS2 for £20k, C6 Z06 is LS7 for £35k. Almost double the price and a huge step up in performance - nothing comes close for similar money unless you buy something dedicated like an Ultima, but that's not something I'd want to daily.
GT-R + Ultima GTR is ~£80k. GT-R can be used daily and has enough performance to not be boring coming from a Z06. Ultima is then for weekends etc.
As above, going from a GTR to an Evo would involve building my own car and doubling the hp.