Welcome to the forum. as far as what sounds best i think that is subjective. what you think sounds good i might not, so for that i can tell you my neighbor has flow master force (insert some number here i think 2 but i'm not positive) on his 12 Cruze ltz and it sounds like a not so obnoxious fart can Honda muffler where as my wife's Malibu has flow master american thunder series on it and it is more throaty and aggressive sounding. as for interior i recommend LED dome lights (quick easy and cheap) and next try vinyl wrapping some of the interior pieces its not that hard. it is an inexpensive way to personalize the interior. I also did the footwell lighting kit from DiodeDynamics great kit easy to install (the only thing i changed on that was the switch i wanted a blue led on it rather than the red it comes with). as always post pics.