I have a very similar party (pala, priest, mage, 2x warlocks). The tank is the 'master' and everyone else is a 'slave'. Here's how I handle healing/debuffs:

NUM7 = Renew target
NUM8 = GHeal target
NUM9 = FHeal target
NUM4 = PW:S target
NUM5 = PoH
NUM6 = Abolish Disease target
NUM+ = PoMending target

NUM0 = target tank macro (by name)
NUM. = target priest macro (by name)
NUM1 = target mage
NUM2 = target warlock1
NUM3 = target warlock2

So I use the numpad to control the priest targeting and healing. It's pretty compact and covers everything I need. I use Trinity Bars to bind the macros directly without occupying a button bar, but that's up to you.

Of course, I suggest that whatever unit frame addon you use indicates party member targets so you can see who the priest is currently targeting. That said, I almost never look at that. I just leave it on the tank by default and switch priest targets when needed.

It works really well for me. I usually just hit the proper priest targeting key automatically before any heal because I don't have to think about anything.

Note that you will need to UNBIND NUMLOCK from autorun to keep from screwing yourself (I use F12 to autorun, but it can be whatever).