Drains: Caverns don't take that long to reach (if you dig straight down, you'll almost certainly reach them before 100z is dug (~equivalent of digging a 10x10 rom), but you might want to not use them as drain. In that case, if you dig to the map edge in stone layer and smooth and then carve fortifications into stone there you have an alternate drain option. A more compact if not as fast drain is a minecart on a track stop set to dump into wall - unlike items, water just gets erased. (Sometimes done with 1x1 hole to avoid water sloshing moving around the minecart).
I use a ramp spiral, but I know some use only stairs as well. Up to you, though it shouldn't happen without combat or sparring nearby.
Your well should be right above water, as bucket takes time to travel. Fisherdwarves can fish there, though if they get anything depends on the availability of pond turtles.
How much water do you need? Well, assuming your booze requirements are fulfilled, you need water for hospital and washing (and mist generators, should you make them). In non-evil biomes there isn't much washing needed, though, so I'd stick with just reserve of enough for "oops I accidentally forgot to brew and caused a mass cavein of ew" - say, 2/7 per dwarf.
Of course, most of the cistern can be far away from well.
If you plan to run obsidianizing elsewhere, maybe. For small things like misters, I stick with bucket brigade. Can also change things later, so no biggie.