Buying new vs. Used is a valid comparison. When i finally convinced my wife (actually, my kids wore her down for me), a used SPF was available not too far from me. I was beaten to the punch by someone who got their money there first. So i bought a new one, right off the floor, i had it in my driveway in less than a week.
In retrospect, i am very glad i paid some additional for my new one. For one thing, in my state, my new one was legally registered by me as a '65, as it came on an MSO, and SC lets us do that only with SPF's for some reason. So i pay about a buck a month on taxes, whereas the one from NC would have been a '99, and by now i would have paid more in taxes than the difference in price was. Plus, i benefited with a newer car, with the upgrades the factory does as time passes. I was able to very carefully break in my new motor, so that the other day i had 173-178 psi on all eight cylinders, and my motor burns less than a forth quart of
oil per 1000+ miles. My new car came with a warranty, too.
There are important considerations to new vs. used, but for me, it turned out to be the best decision to go new. I believe SPF's hold their value about as well as any car does, although that part of the equation was never of interest to me. I have never gotten "tired" of my car and wished for something else.
Now if you read the Porsche forums, there you will see the laments of depreciaton!! I think if you make the right decision for you initially, you won't worry about depreciation either.