Don't go for Mehran or Santro.
As your mom will drive it, she will say to have small car & can zig-zag the car in heavy traffic with no hassle, my advise is to buy a used 06-08 Coure Auto Petrol. Your mom will like the drive of an auto tranny which is easy than manual tranny plus performance on petrol is awesome & A/c is also good with no over heating issues & overall the parts are not quite expensive for the maintenance.
If you want CNG then you can install the after market CNG in it.
Or if you can afford a brand new car then buy Daihatsu Coure Eco-matic. Your mom will love it.
If you want a manual tranny car with some more performance then get an Alto instead Coure.
About price tag, other senior members will help you.
I hope you understand it!
Abbas Ali Abbasi