I've highlighted the issues i think are on the chop as for now! The base, as you've mentioned, isn't easy but i think if you're gonna take on a car like this than be sure to do the basics perfectly...changing a lot wont make a chop great! Do the basics perfect to have the overall look then anything after that is details!

The blury bit behind the window need fixing, too toony and lacks reflections of the environment! The scoop you put on the roof cuts into the window seal, its at a wrong angle, and finally it doesnt match the curves of the roof line! Though others have helped already with the background you've still gone off to ignore that and use a different background...thats perfectly fine BUT, now the car is off at an angle to the asphalt...shouldn't take long to fix, just rotate the car a little and it'll be fine!

Keep posting pics soo we can all help!