Some really interesting points about future values and what might influence that value. However, before thinking more on that, it's probably worth reflecting on the fact that almost all of us have been fortunate to buy our cars at reasonable prices. We are wondering about what will happen in a few years time when, hopefully, the cars have gone up even more. Let's look at 5 years time and say there is a difference in value of £5-10K between a 100% original and a car with "sympathetic" upgrades. My first reaction is that the upgrades have cost an additional £1-2K per year BUT, this is appreciation, not devaluation and I am sure that the increase in value will still be very pleasing! After all, I don't think many of us have bought the cars as an investment.
Therefore, I think for most of us, MoTec +1 is not an issue. Let's say I bought a car for £38,500, spent £8000 on Motec +1, kept it 5 years and it was worth £65,000, I would be happy, even if a totally original car was worth £5,000 more.
The issue is for the guys spending £45-50K on a car. For them, it is harder to justify doing this sort of irreversable work.
As for Chris's question, that is a hard one to answer. If it had that engine fitted 10 years ago and the car has an interesting history, I think it is OK. GT Classics has such a car, that has been extensively upgraded, but it seems to have taken a little longer to sell (more than the nanosecond that most of his stock lasts
). However, I suspect that if it were done today, it would effect the value of the car. Having said that, if the engine is genuine, I suspect you actually have a rather valuable engine! If somebody has a 3.8 964RS which blows it's engine, this would be what they need to preserve a lot of value. I would guess that the difference in value between them fitting a genuine 3.8RS engine and a rebuilt engine to that spec would be a fair amount of money, say £10,000. Add that to the cost of rebuilding a normal engine and you could easily value that engine at between £15K and £20K, if genuine!!! (Just my opinion)