Q1: No, "float" is the amount your feet can rotate (around a vertical axis) while clipped in.
A system/cleat with little or no float keeps your foot in a very rigid position. You can't freely twist your toe/heel while pedaling. If you have a textbook pedal stroke, low float will keep your feet in the right place so you can focus on other things. With more float, your foot has more freedom to rotate. This can be helpful if your technique is less perfect and your body needs the extra freedom. I have an old knee injury and need plenty of float -- with less my knee really hurts. So you want to get enough float to allow you to pedal comfortably, but not so much that you develop a sloppy pedal stroke.
It doesn't really affect how far you need to turn your foot to unclip. Well, you do have to turn further in a system with more float, but that's not the point of having more float.
Q2: Pedals normally come with a set of cleats and bolts to attach them. And most shoes are designed to fit a wide range of cleats and offer a fair amount of cleat adjustment. But if in doubt, check before you buy. You probably want your LBS to help you get the cleats positioned correctly anyway (I've done it myself but it involves a lot of trial and error).
Q3: My experience is limited to Crank Brothers and SPDs -- both of these offer different clips for differing amounts of float. Don't assume that all systems do this, so check them out first.