Of course, I'm a little biased but I think the Ford Racing Puma has potential to increase in value. A decent one is currently around £5000 to £6000 (mint one go for more), and at least 2 are already in museums.
It has the key ingredients of low numbers (500 built), beauty, magnificent noise and a dramatic (Sparco blue alcantara) interior
It's estimated that only about 250 of the original 500 cars still exist - it's difficult to be precise as DVLA records do not differentiate between Racing Pumas and ordinary ones.