Hello an welcome! I guess you don't have the Interscope sound system on your 595c standard? That's actually a pretty good system with the sabelts (the sub goes under the drivers Sabelt) but not too sure on getting it installed after you have bought the car, a dealer would probably be best to contact about this, otherwise custom installs would be the way forward with that.
As for MPG, I am pretty sure you will be getting more than 29mpg on the 595c, it's more like 35mpg combined, the tank is 30 Litres with a 5 Litre reserve. Not very big, but I'm pretty sure you can get 300+ miles from a single tank if driven sensibly.
The Abarth has a lot of character, something that's missing from a lot of euro compliant obsessed cars these day that are eager to please reviewers. I am pretty sure you will absolutely love it.
Looking forward to the pics and again, welcome along