1. Depends on the hotel and the driver. It's almost always going to be cheaper directly, but at some hotels, the difference won't be extreme, others it will. Basically, the more expensive the hotel, the more higher the difference.

2. You can hire someone off most major streets with no notice. I would give at least a few days notice if you are trying to hire some of the drivers recommended in this thread.

3. Some are awesome, some don't know anything, some will just make stuff up If you're looking for someone who is also a really good cultural and historical guide, do some internet research and book beforehand. If you're just looking for suggestions on things to see and do, most all drivers will be able to handle that no problem.