993 Turbo/Turbo S, CGT, 930 Turbo's, 356, if you get a good vehicle for a good deal, someone may be willing to give you more than you paid for it due to them really wanting a mint vehicle, but in terms of appreciating to get a return on your investment, your in the wrong field, vehicles are meant to be driven and enjoyed, you will always spend more money on them than you will ever get back from them, invest your money another way and enjoy the Porsche you bought.
Here is a good buyers guide I ran across a while ago.
Here