Is the Ranger a new class or do you mean a Hunter? Would definitely pick a set of toons that can handle each role specifically for the PVE LFDungeon game mechanic. If you are interested in doing PVE for a while that is...

Say, a Druid (balance) to fill the tank role, S.Priest (to fill the healing spot), and then Hunters, Locks or Mages. They all have good ranged, all have a CC if needed. Locks have a benefit of being able to summon, whereas Mages have ports.

If this is your first attempt at MBing, perhaps my advice would be to start a little more simpler, until you become a bit more familiar with the multiplexer you choose. I would recommend 5 druids... you got tank, heals, and dps all in one "ranged"-able class.

Once you get the general feel of it, you can start introducing a completely different class.

If you are interested in PVP mostly, well then you can omit the option for tank and healer, but definitely go 4+1 heals... and your options are wide open.

Pouring over the new to MBing section for a good... and I mean good long while wont hurt you at all.