Unfortunately on the FAQ, the RPO guide is only for 2003 and Parts list is for 2003/2004. The other RPO lists are for the CTS-V. The PDX option was for the sport suspenstion, without having to get the full-up 1SC package, for 2005. I believe it comes with non-load leveling rear sport shocks, since you didn't have to get the HID's to get the PDX (if anyone knows differently, please chime in here, since I'm going from memory).
The sport shocks and springs are different from the lux and base models, as well as the rear sway bar. I think the front sway bar is the same on all CTS's (other than the V). A trip to the dealers parts department may be in order to get the correct part numbers, unless someone here has access to their database.
My guess is that new PDX parts will run ~$ 250 for a set of springs, ~$300 for the shocks, and $100 for a new rear sway bar and bushings. Your best bet might be to find a used set.