1) I haven't seen a double switch for 1 box, but if it's available, I'd say use it. If you can't find one, you'll have to add a switch. You can either replace the existing box with a double box or add a second single box. Adding a single can be easier because you don't have to disconnect the existing wiring and remove the existing switch and fight the nails in the stud to remove the old box. But it's not as neat when you're done. It looks nicer to have 2 switches under one plate on the wall.
2)Some people just add a single hot wire to the new switch. I'm not sure if it meets code but it's functional and reasonably safe. If you can readily replace the existing sheathed electrical cable without tearing up your walls, that's preferred. (If you do replace the 3-wire sheathed electrical cable with 4-wire, then you'll certainly want to go with both switches in 1 box.) If you add a single wire instead, a 16-gauge is sufficient. Neither the fan or the light kit uses much power.
3) Wiring is not complicated. At the "hot" side of the existing switch (that's the side that's not affected by operating the switch - usually on the bottom) attach a short black wire from it to the bottom (same side) of the new switch. Now, both switches have power available to them. When the switch is ON, power can go through the switch to the wire at the top of the switch, which goes to the fan or light. One goes to the hot wire (usually black} wire of the fan, one to the hot wire (often blue in color) of the light.
Neutral is not switched, and only one white wire is needed to go to the white wire(s) in the fan/light unit. Same for ground - only one.
4) You might like to do what I do; use a dimmer switch for the light instead of an on/off switch. The wiring is no more difficult and dimmers fit in the same box as other switches.