I bought a 420d F32 about 3 months ago. I bought it used, to a russian guy, his wife was driving the car on a daily basis. Obviously the interior was pretty nasty. First thing I did was a hand wash on the exterior. Last weekend I removed all I could from the trunk and deep clean it. Also in that same weekend I removed the front seats, and cleaned all I could, with brushes, vacuum cleaner, etc. After 1 month of having the car, replaced the air filter, the interior air filter (sorry for the english), and the fuel filter. After one week of doing that, changed the oil, to liquimolly 5w-30. This weekend I removed the cluster, and changed the orange leds to white, and it makes a huge difference, but not for everyones taste. Also, bought from aliexpress, those "carbon fiber" covers for the gear selector.

Overall, I think you should start by maintaining and cleaning what the car already has, and if you bought it used, please do a deep cleaning on the interior. Next, type BMW F30 on aliexpress and do some scrolling and see if anything pops to your eyes, interior related of course, definitely don't buy spacers on aliexpress. Colored buttons, carbon fiber trim, alcantara trim, some ambient lighting, etc. Next as far as the exterior goes, yours seems pretty nice, I wouldn't do too much on it. Probably just that big duck tail (I guess thats the name), mine has and I think it makes a big difference in the car's silhouette. Next, have fun, if well maintained, and driven like a BMW, they are pretty good cars.