The price on the 320i will be based on the model and spec Exclusive, Msport, etc
I would rather go e90 323i if she can, if the E90 325i is out of reach, the inline six are far better engines, the 320i fuel consumption is not the best either, small engine and heavy body to pull.
If she has to buy the 320i, check he cars history at BMW.
Check for excessive smoke on startup and under acceleration, 320i’s have valve stem issues.
Common E90 issue are the
Water pump: I just spent R9500 replacing mine
Oil filter housing leak (easy fix)
If it’s a MSport check the suspension
The diff is also a weak point. It may last a bit longer on the 320i