I'd replace the spark plugs yourself, they arent super hard and that will most likely fix the P0300 code.

The P0420 is the converter going bad and you can just replace the converter section, you should not have to replace the whole exhaust system as that is just crazy talk. I am not sure if AC Delco sells just the downpipe section without the catback. On a 3800, you can just buy the downpipe with cat converter.