You do need to decide how you want the fan controlled?
e.g.
(a) Just come on while the light is on?
(b) Independent switch so you can turn it on/off irrespective of the light function?
(c) Additional run-on timer so that the fan stays on for a short time after the room has been vacated?
Assuming you do have a working light in there already then all of the power requirement you need are there, but as Murdoch says it can often be a bit tricky accessing them and then getting cables to the fan, especially if they are under the stairs!
Unless you are happy with that miniature surface mounted PVC trunking?
Some people just opt for a box a matches and a scented candle on the window-sill?