Go for the 325 , 320 has no power . I have the e90 325 bought it 2 years ago second hand . Beautiful car to drive handles very well.
But servicing costs can go up very fast. Normal oil change is $200+ .
But once the car does 120k then the wear and tear all come in . I just changed the front bushes , rear brake pads and rotors , battery , light bulbs. Air con now not cold and my exhaust solenoid for the vanos needs replacing . You can say good buy to $3k for all .
My advice is to get the car checked before you buy it.