There is probably a certain group of 70s/80s and 90s cars that will always be classics and collectable. But going forward its hard to see the more modern cars been collectable and restorable
Nick your quite right in your summarisation that "The 1M is a definite future classic, all of the others are being built in much larger numbers than the first and second generation of M cars so there will be less exclusivity which will probably effect prices, but in 25 years only a very few will survive as minters and they will be the cars that will sought after by the current teenagers"
I think I might give up on the e30/e28/e34/e36/e46/e38/e39's and start looking at the newer M's before the young lads snap them up!