IMO, Realistically if you get £6k for it with the R154 I'd snap their hand off. But even then I would think you are in for a long wait. With regard to breaking it, I think it would take a long time to get near the money you are asking. It would be a different story if the car had a 6 speed and a 2JZ-GTE much like IDO is selling. As these "big ticket" items would bring in the majority of the money straight away, circa 5-6k on their own. But a R154, W58 & 2JZ-GE altogether I'd recon would fetch about £2000-2500. Then you would have your turbo kit and everything else but I honestly don't think it wouldn't net you anywhere near the extra 5k that you are after. As essentially all you are left with is a modified N/A rolling shell and a turbo kit.
Just trying to put a bit of perspective on it, as although we spend fortunes on these cars, you are only likely to see a fraction returned when it comes to selling them.