Karachi-Quetta: Road condition is good except for the last ~one hour stretch before Quetta.
Quetta-Ziarat: good condition
Overall, its doable, imo. Car will give you more flexibility especially while roaming around Ziarat.
If I would take my car, I will try to do that in one day from Karachi to Ziarat. This might be difficult but can be done if one leaves Karachi very early in the morning, say 5am, and does not stop unnecessarily. The hotel at Khuzdar where Al-Aziz coach stopped was quite reasonable for half-way stopover and had cleaner toilet facilities, which is very important.
Do give me a PM when you finalize your plan, I can share observations in detail.
Hope this helps
Muzzammil's Travels: Chilling Around Quetta and Ziarat
