Buzyg has got the right idea as far as I'm concerned. I own an M Coupe because I love the car, not because I couldn't afford to go out and buy a new M40i or equivalent. So if that car fitted the colour/spec I wanted, why not? It would certainly be on my potential to-buy list.
I do about 3 - 5k per year so it would take years to put significant miles on, but even if someone bought it as a daily driver, surely the depreciation wouldn't be anywhere near as much as a brand new Z4 doing the same mileage. It's probably £8k - £10k more than a really good one with 50k on the clock. But that extra money would buy you the peace of mind of knowing exactly how it had been driven and being able to look after all the weak spots that have been discovered over the years, and in 10 years, even with 50k miles on the clock, it would still be worth more than the new M40i with equivalent miles that you could have bought today.
I guess the only disadvantage of this car, over a new one, is that it has been sitting undriven for quite a while and if I bought it, then an inspection 2 would be the first thing on the list, and I may be negotiating that into the price.
We are all a bit guilty on this forum of looking at our cars as future classics, but I think this car could justifiably be bought and enjoyed as a new car, not as an investment.
Now, where is my chequebook?