Dave,
You could do far worse than to purchase a copy of "Theory of wing sections" by Abbott & Doenhoff. Don't be put off by the maths (which looks daunting to all of us) since there is a good section on lift / drag for various aerofoils currently in service.
Theory of Wing Sections: Including a Summary of Airfoil Data
The ideal for any aerofoil (to minimise lift to drag) is to operate as near to stall as possible. In practice the need to allow for manouvreing, payload and a practical flight envelope means blades seldom achieve this. Thicker choord blades are more suited to reduced rrpm, but should work just fine. Ultimately as long as it produces lift it is just a question of how to maximise performance / battery life...