Usual mod for the P100 if you want to go fast is a transplant from a Sierra or Granada 2.9i V6 or 24 valve V6 from the Granada / Scorpio Cosworth. Don't see two many 2.0 4 cylinder Sierra Cosworth conversions as they still fairly expensive. With a stock 2.9i 12 V6 it works a treat. 150 bhp / 173 lb ft of torque at 3,000 rpm takes care of lugging the 400litre tank about, is pretty much indestructible and in fuel injected form should still do around 25mpg which considering my 1.8TD only manages about 29mpg can't be bad. If I didn't need the extra seats I'd go down that road with mine but finding a 3.09:1 Capri rear axle isn't easy and you need it if you don't want to go deaf at 60mph.
Fitting a Rover based TVR unit is a brave move though and with the Capri axle and RS 500 bits is probably worth more than the asking price in pieces.
Might have fun trying to commercially insure it though.....