Alex, I think every now and then a car comes out which just sort of screams 'Future classic', alot of it's down to the engine, and alot of it down to the body, say for example the old VW Corrado's, they come with the G60 lump and the VR6 lump, both cracking lumps that help the car to become more of a classic. (Of course they come with crappier lumps, but the decent engines stand out from the crowds when it comes to classics)
Perhaps the early edition sport pack V6 156's MAY hold some decent value in the future??? As you say it's hard to know for sure. But in my opinion the 1.8 Twinny engine won't help any 156 become a classic, am I wrong here? Does the engine matter at all when it comes to classics???
I think to an extent though, if you hang on to ANY car for an extremely long amount of time, look after it, and don't put too many miles on it then sure, maybe one day it will be worth a bunch, ya never know.
I have a 1994 Mondeo Ghia 2.0i, it's pretty much MINT condition with 44,000 on the clock and FSH, is it a classic..? Hmm, no not really, it does look funny though being a 16 year old Ford in spectacular condition, but does that make it worth any more than £500? Not sure, but I love that little car to bits...