Let's do the easiest first. That would be the serpentine belt. This takes about 15 min. Buy a new one from our host. Remove the air filter box (13mm). Use a 24mm socket with breaker bar to relieve tension from the idler pulley, pull off the old belt and put on the new. Take a picture of the way the belt goes over all of the pulleys before you remove the old one. Here is a link to the complete procedure from the RennTech 996 web site (you may need to become a member to access this):
http://www.renntech.org/forums/index.php/tutorials/article/57-changing-the-polyrib-belt/
Be aware that in the MY2003 Carrera, the OEM Porsche stereo was changed to a fiber optic type connection ("MOST") system that may not be compatible with non-Porsche after-market replacements.
No comment on the radar detector.
Good luck!