I reckon it would be more than £50,000 if done right. I seen Mountune advertising WRC spec engines for £32,000, Millington Racing Engines can charge up to £35,000 and more for their engines too.

My idea:

£10,000 for the car.
£5,000 for chassis strenghtening, roll cage, uprated bushes, strut braces and roll bars etc.
£5,000 for a serious suspension kit and full geometry setup.
£5,000 for good wheels and big brakes front and rear.
£35,000 for a Millington racing Diamond Series Engine.
£5,000 for a full hybrid turbo.
£20,000 for a full Quaife transmission including gearbox, front, centre and rear differentials.
£5-10,000 for other parts such as cooling system ( intercooler, radiator, possibly chargecooler separation kit and swirl pot.), uprated hoses and stainless steel pipes,nice bucket seats and harnesses, full gauge set etc.

I rounded some things off to the nearest thousand, but all in that comes to around £90,000 odd!!!