Beware of speed controllers! They're not all the same. I learnt the hard way.
I got near silent buying a super oversized 10" regular RVK fan, and using a variac (or voltage regulator) to reduce speed to 1/5th (2/5th late flower).
Regular rheostat (I believe) style speed controllers are much cheaper (and extremely lightweight), but often will make your fans hum like mad, and when I mean like mad, I really mean like it's not even worth using. These are usually the one's with built in thermostats.
These crappy speed controllers also seriously reduce the lifespan of your fans, while variacs will preserve them.