Buying a car with an eye toward it's potential investment value is an extremely tricky thing, even for folks with a lot of experience. The main factors that determine whether or not a car will appreciate include:
Numbers produced, as stated by others above. The fewer there were, the better;
Price/exclusivity. If they were cheap and readily available when new, the market's probably flooded.
Beauty/performance. Helps to be widely recognized for both. Cult cars like Jensens or Bricklins aren't worth much.
Historical significance. Racing history, show car or normal production vehicle produced during the time of a personage such as Enzo Ferrari. Celebrity ownership sometimes helps.
Cachet. Some cars like Ferraris produced until about 1970 have it. Others never will. Extremely hard to be unemotional about and harder still to predict.
Even worse, now that everyone's wise to all this, every Corvette Indy Pace Car Special Edition is either way overpriced or the current owner is badly upside-down andn regretting he ever had the idea to have a car as an investment in the first place.
A few years back and certainly well before the stock market melted down, I saw a comparison chart of the price of a Ferrari 365 GTB/4 Daytona coupe, compared with some stock, bond or other paper investment, over the same period of time (decades). And since the chart had been prepared by some investment firm or other, it naturally showed thatthe paper investment had the superior return. But then again, you can't take your bond portfolio on a blast up Pacific Coast Highway through Big Sur, either.