If you have not driven a car in Pakistan previously but do need custom rides, get a car with a driver. You can always test drive but don't have to worry about parking and stuff. Plus it doesn't cost that much to find the driver there anyway.
its quite easy to rent a car with a driver in karachi these days.
I rent one everytime i go (cause' i'm too scare to drive !)
prices around 1200-1500 PRS per day (if block booked).
dont know of any websites but whn u get there just ask family/freinds.
Avis Car Rental is also operating in Pakistan. Of course they are quite expensive as compared to local rent-a-car companies, but if you have an Avis Discount Card that might be an option. You could even book online on their website http://www.avis.com.pk