The consensus is that copper is best, here's a thread to read about the plugs for the Turbo....
Copper or platinum plugs for turbo
As far as the dual exhaust, you don't have to worry about spending anywhere near that 3K you mentioned. Even the custom side exhaust with leg pipes only run 1K. Everything made for the PT comes out the CAT and then splits so we're not talking real "dual" exhausts. As Hooligan already said, it's best if you can hear someones set up first if possible because what's music to some may be noise to others.
There's quite a bit of choices for exhaust such as, BTG, Borla, Hearthrob, PTeazer and Trendsetter. Here's a couple places to give you an idea of what they look like and the cost.....
Chrysler PT Cruiser Exhaust Systems - PT Cruiser Accessories - AutoTruckToys.com
Chrysler PT Cruiser Exhaust Systems - Chrysler PT Cruiser Performance - PT Cruiser Accessories Store...