For the e90, check on maintenance esp with regard to the transmission and fluid changes. If you have no service history, walk away.
Common problems include the fuel pump failing past 100k, sensor failures and transmission failures due to lack of maintenance. This gets expensive.
Rarer problems include vanos problems. The aircon is known to have intermittent electronic issues
Usual maintenance at 100+k include seal changes. The car needs preventative maintenance and if you aren't familiar with cars, don't touch a used BMW.
At this age, you'll be looking at bushings and mountings changes. Spark plugs may be due for a change as well.
On average, for an older e90, expect to pay about rm10k on maintenance per year excluding fuel, insurance, tax, tyres. Total incl tyres is closer to 15-20k.
If you can't come up with this kind of money without blinking an eye, don't bother.
However, if you do your own wrenching and know how to source parts- ie home mechanic, then go ahead. This bill comes fown to about 4K. But keep a second car for the months, yes, months, that your car is out of service.
I suggest you keep a used myvi or Axia around.
I deal with older cars and keep two extra cars because at anyone time, something is in the workshop for routine or preventative maintenance.
Buy a cheap old Bmw still kept my Proton Saga FLX for spare or emergency used too