welcome to the family! As for sub system, if you want to keep cheap walmart is your best friend. Thats what I did, 2 12" Sony Xplode subwoofers, 1000 watt Sony Xplode amp and all the wiring and enclosures was $250 and 45 minutes of time. If you have any questions dont be afraid to ask or snoop around!
Thanks for the compliments! This car continues to amaze me. I have averaged over 34 mpg consistently, and just yesterday averaged 40, cruising between 60 and 90 mph! No joke. I thought the calculator was off so I topped it off and calculated myself. Simply incredible car that I would recommend buying to anyone.
Audio depends on what you are looking for, how much space you want to give up and how much you are looking to spend.
Other parts. Like said not alot of aftermarket, but doing your own stuff can go along way.
As far as mileage. If you calculated yourself and got that amount great. My xD averages between 28 and 32 depending on how I drive or my wife drives. The average listed on teh instrument panel in normally off by about 2 to 3 mpg.