Since the iPad uses a USB charging port, the voltage is fixed. Unless you buy a non-USB charging solution for the iPad (if any exists) you do not need to worry about the voltage. It will always be approximately 5V. Even then, if it is designed for the iPad or other devices normally charged with a USB cable, it will still be 5V. It's a standard.
While an iPad may charge with only .5A (2.5W), it will do so very slowly and only when off.
Yes, it should not be a problem if the power supply is more than 2A or 10W. The iPad may even charge a little faster, but not a lot faster, since as you point out the maximum amperage draw is set by the iPad, not the charger. This is why it is perfectly safe to charge your lower powered iPhone on the iPad's charger.