The panhard brace firms up the suspension in the rear.
If you already have the other parts installed, it's not really worth buying it and then paying someone to install it. However, I think it's worthwhile to do at the same time as the other mods--its essentially a free install while everything else is apart anyway.
As for the adjustable LCAs and the relocation brackets, they are totally seperate parts from the panhard and UCA/LCA. If you have an adjustable panhard and/or adjustable LCAs, that's great, but that has nothing to do with the LCA relocation brackets (and vice versa)
Adjustable LCAs allow you to change their LENGTH and therefore the pinion angle of the car. The LCA relocation brackets let you change instant center. Those are two totally different things. If you want to adjust both, then you need both parts.
read this thread:
http://mustangforums.com/m_2230421/tm.htm