if you intend to dry washing in there, you really do need an electric extractor. Otherwise it will get very damp. In most circumstances (e.g. bathroom or WC) the light switch plus an overrun timer is best, as it assures ventilation without any effort. However if you plan to put the washing in and leave it to dry, I suppose you had better get a manual switch.
A typical extractor fan will run for about 50 hours on 12p worth of electricity, so it does not matter if you forget to turn it off.
Modern ball-bearing fans are very quiet and last a long time.
100mm/4 inch fans are very weedy, so look at a six inch/150mm fan.
This brand is very quiet and has a good reputation. It is about three times as powerful as a typical cheap bathroom fan.
Mount it as close to the ceiling as you can, because water vapour is lighter than air, and rises.
If you think that one day you might buy a tumble drier, have a 4-inch hole made about 600mm above the floor, with a hit-and-miss vent on the inside. It will also allow fresh air in without sucking it from the house. Remember to have a power socket nearby.
Put a cowl vent on the outside of both ducts. It does not rattle as much as the ones like a miniature plastic venetian blind. Six-inch ones are hard to find.
An electrician can fit it for you. You might like to have the switch outside the door with an indicator neon to remind you (if so, specify it before the cable is run or it will not work)
You can get a speed controller, but it will cost a lot.