A year ago i was in your shoes, and i had an E90 too but i chose the C180 over F30 due to the headache of Bavarian.
I was at bavarian recently, and i found out more than a car 320i F30 "kasr zero" for 350 & less, also Mercedes authorized used dealer has more than a C250 car with same budget.
The C180 has less options maybe than F30 & has a tight backseat but i love the car.
So, go have a look at the agencies for the certified used cars or even Online through their websites
BMW Used Cars Young Stars Certified Pre-owned Mercedes-Benz