How powerful of a drill do I need?
12 to 18 volts is most suited for cordless drills, and 6 to 8 amps for corded drills for home use.
One battery or two?
If you need a cordless drill for smaller jobs for shorter periods, one battery will be sufficient. If you, however, intend to work continuously you should consider getting an extra battery for your drill.
What is the difference between a drill and an impact driver?
An impact driver has more torque than a standard drill. Impact driver is used for driving large fasteners, while standard drills are used to drill holes and to drive small fasteners.