My friend took a trip in his City 2010 to Ormara and back in January 2011. He took along two 20 litre jerry cans of petrol with him. But nothing else as his car was brand new. But since you have a slightly used car then I would suggest you take along general maintenance items as the people have suggested.
One reason why I am against your trip in this month is the heat. In hot weather engines tend to heat up. I would suggest not using the air conditioner. Using A/C would put additional load on the engine and consume fuel faster. Thats why a trip in winter is recommended.
In addition, no mobile service is available along the coastal highway except near Karachi and at Ormara. If you have a Thuraya you might take that along in case something happens. But buses pass along sometimes so you can ask them if you need any assistance.
And most important, please drive safely!