Put the cash down on the newest one you can make the payments on with Porsche Certification. You'll end up with a much better car and will probably spend less over the term of the financing/warranty. (notice how I linked those terms?)
The only way I would do it another way is if you plan on doing a pile of mods and can't live with a normal, reliable, fun roadster.............
Just buying parts to do your own work on something older that's going to
need more work, adds up quickly when they have those little Porsche part numer tags stuck on them.
edit - part numer, does anyone know what a part numer is? Damned bear paw fingers!