Class makeup is fine. You don't need a dedicated healer until you start heroics and maybe not then. I strongly suggest having your tank as your main window since most of the action goes on there. Your priest will be the simplest to heal with but any healing class can work.

With my group I went disc on my priest for two reasons. 1) I couldn't get a shadow sequence that could keep pace with the rest of my team and 2) I could do decent dps and keep the tank going with Shields, Renew and the occassional CoH or Holy Nova.