Go for Mehran. It's resale is not a problem. Use it until next summer. By then you will have idea of using and maintaining car. In summer if you want a car with better AC, you can sale it.
For buying car, if you can find a non-professional seller i.e. someone selling his own car, it will be really good. You can search online forums for it.
Otherwise visit local market, find car, check it's physical appearance (both out side and inside the car), quality of the tires and observe it's sound when it starts, time it takes to start and smoke emission.
If these tests are passed get car for a test drive and find a mechanic in area a bit away from the selling merchant area. Ask him to examine car for you in return of money (300-400).
After this stage is documents verification.