Before you go shopping, decide how much you are willing to spend. Then do yourself a favor and don't drive anything above that limit
Also think hard about what kind of options/colors you would like and focus on those cars. There are plenty on the market, so you can afford to be picky.
There are so many used 986/987 out there, that trying to find one that will hold it's value is not too likely. They will all depreciate, even the special editions.
And spend some time reading the info at:
http://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages