Edit: My first BMW was a 316i! I just upgraded as and when i could afford it so i completely understand the OP.
What my advice was, and still is, is to get a 4 pot diesel. Reason being when you get used to the power and it seems sluggish (trust me this has happened to every bmw i have had, including my previous 320D which had a tuning box), you can easily boost the power. However IF you don't care about power, or its unaffordable, get a 318D. This will also respond good to remaps/boxes.
But if your heart is set on the 320i then good luck.