Salam. I came to Lahore last week and may be able to answer some of your questions.
1. Route should be Karachi to Hyderabad via motorway M9 and then onwards to sukkur on the National highway N5. Meaning you keep going straight crossing Hyderabad on your right. From sukkur you can take the motorway to Multan M5. Then onwards after khanewal to M4, which will connect you to the M2 taking you to Islamabad.
2. Sadiqabad and RYK both should be ok. I felt Multan was a bit of a stretch for me with family as I stayed there. Multan would be fine if you don't get stuck too much on the national highway, which I did.
3. I stayed in Multan but RYK or sadiqabad should have hotels within any budget.
4. A new car should do just fine specially if the 1000 check has also been done. Takes care of any teething issues. My car is a 4 year old one but I got oil changed and overall inspection done before leaving. For the car I kept some water in a can, a battery powered tire pump, checked all tools and the spare pressure as well. I think that was about it.
5. License, original car papers, and if owner not travelling then an authority letter in the driver's name. Nobody stopped me for any papers or checking all the way to Lahore.
6. Depends on the network you are on but my Jazz worked almost throughout and Google maps was a help but better to plan ahead on the map and also follow road signs. Last min direction changes on the motorway can be dangerous.
Hope this helps.
