Rear disk brakes To do or not to do?

Given the weight of an A-body, rear discs are a luxury not a necessity. If you have some expendable cash, I would go for it.

If you do the install, don't short cut it. Get the right MC, proportioning valve and any other related components needed to take maximum benefit of the new system.

With all that said, unless you are auto-crossing or do road racing track days, you will more than likely never see the benefits.