Carnets for your vehicle can be obtained from 2 organisations in Lahore and Karachi respectively (forgotten which ones but info is available online and in Pakwheels forums). They require you to submit 80-85% of the value of your vehicle and issue you the carnet. On return to Pakistan the amount is refunded.
Chinese tour guides info is also available online and costs are probably available on request. In my opinion it will be expensive as per day charges of guide, food, accommodation, air tickets to and from Pak China border to guide's hometown will be quite a bit.
Five days will be too less on road. In 5 days you will barely reach Urumqi (which is around 2000km from the border) in the far West of China. Minimum time should be 2~3 weeks after entering China.
Driving through China is not easy primarily owing to difficult procedures in place by the Chinese government. You will definitely need to hire the Chinese tour agency to sort out documents on your behalf as I have lived in China and i know a bit about their troublesome attitude and procedures. Good luck! 