I have high blood pressure and heart rate. My doc is about to put me on Zoloft (sertraline) 50 mg. I read Zoloft (sertraline) can cause qt prolongation. Will I get this if I start the zoloft (sertraline)? What are the chances?

Dose-dependent. Many antidepressants can cause qt prolongation in a dose-dependent fashion. Presuming you do not have this at baseline, you aren't likely to develop it at 50 mg of zoloft (sertraline).
Unlikely. I've had tons of pts on zoloft (sertraline) some with heart problems--none had to stop zoloft (sertraline).
Zoloft (sertraline)? Zoloft (sertraline) is one of the safest medication for depression and anxiety . Not everton side effect taht is listed about medication , mean will happen to you , I have written this medication for many people none of my patient had the side effect that u mentioned.
Rare. Millions of people take this medication daily. Many overdoses have been reported without significant cardiotoxic effects.