"how loud...?"
It's not. It just kinda hums a little. the most noise mine ever make is if I let the water get below their output. Then they splash and make a water sound, but I think that goes for any filter. That and if the intake is over your airstone, they will pick up bubbles and make some noise.
The 100 watt will be fine for your tank, but you may want to consider going with the next size larger in the bio-wheel filter. I always double my tank capacity when choosing a filter. You can't over-filter, so I just go for it. It clears the tank up real fast whenever I stir up the gravel or anything.