In general retention of value over time means keeping the car as original as possible. If you plan to buy a 993TT the more it's driven and modded the less it will be worth down the road, and it certainly won't appreciate in value as an all original example serviced only with OEM parts. Needless to say any accident damage will severely affect the long term value. This is why it's important to determine in advance what you plan to do with the car prior to a purchase, i.e. collector vs. driver. A lot of people make the mistaken assumption that is is possible to do both simultaneously.