I have been travelling on this route (Khi - Lhr - Isl) a lot and I totally agree with @a_n_memon88. Forget about 120+ speeds atleast in sind. Try to leave Karachi at 4AM and keep a steady 100 after Hyderabad. Keep your eyes on the road and be carefull when you reach any 'pully'. There usually are 'khadday' at both ends of 'pullies'.
And it usually is not possible to achieve average speeds of 70+ km/h. Keep lower 60ish as your estimate and you / your family / your car will be fine and you will enjoy the journey.
Keep a print out of this distance table for reference
Distance table - N5
I will be travelling on 17th but since I am not with family and will be with another good driver, I will leave Karachi after office hours and will be travelling through the night. Not a good idea I know but have to be in Karachi on Friday and in Lahore on Saturday.
If you still have any confusion, feel free to call on 0 three three 4 - nine 8 6 0 three two seven. Or if you happen to be around clifton area we can sit together. Aram say samjha doon ga sara raasta