Ohhhhhhhhh Mr Hayward
I can see the appeal of having something like the Ultima; you and I have looked at these many times ( You with a view to owning and me with a view to just having a look

)
If you Evo replacement needs to do the same job as the Evo, then I think it's the wrong car. If the Evo replacement needs to woo customers and demonstrate what APT can do with preparation then I think you'd be better off with something like a BMW or Porsche. Performance derivatives for both are falling into sensible money for and there are no really dominant specialists for either brand in the East Anglia area. The closest rivals would be BMsport in Bexley, Thorney in Milton Keynes and JZ Maztech in Kings Langley. (By rivals I mean motorsports and performance road independant specialists)
If you were to develop a 'proper' E46 or 996 I think you would have some huge potential for further sustained business in your preferred field. Both of these client fields seem to be happy spending decent money on management, suspension solutions and the like. Both cars can be very competitive too and they both have an aspirational quality to them for owners of most other performance brands. If you were to look at it the other way, how many GT3 owners would want an Japanese tuned car for the same purpose as their GT3?
If your Evo replacement only needs to serve the purpose of putting a bloody great smile on your face then, of course, the Ultima will do it. As your desired use seems to be circuit orientated though why not go for something that is more likely to give you the real buzz you crave? Something superlight, high revving and blisteringly quick. Something like a 1'09 is regarded as nothing special on slicks for a Radical.
I think something that can be fun, forgiving and a step towards motorsports would be good for you. If it's more modest as an investment you can change your mind without having a fifty grand burden too.
A lovely position to be in mate. Have fun chewing it over and making your mind up