There are some long, good existing threads on this that you'll want to check out.
Summary: most reasonably cold hardy breeds, well-managed (meaning, most importantly, in dry draft-free air, meaning, GOOD VENTILATION well-located in your coop), are good well down towards 0 F and often quite a bit lower.
A lot of frostbite seems to result from people shutting down ventilation in an effort to keep chickens warmer, with the result that the temperature in the coop is a *little* warmer but it also gets real humid and humidity+anythingbelowfreezing=frostbite.
You might want to take a look at my ventilation page (link in my .sig below). Other than that, it is never a bad idea to have electricity available so you *can* run a bulb for heat if you should *need* to, but in Maryland, you would have to be doing something wrong, or very unlucky (e.g. chickens get sick of something else during a cold snap), or have very non coldhardy breeds, to actually *need* a lamp.
Good luck, have fun,
Pat