PWM stands for pulse width modulation. I'm not too knowledgeable about this, but I'm pretty sure they control RPM's based on your CPU or other hardware's temperature.
That depends on the heatsink. Heatsinks with wider fin-spacing will work better fanless. I would just plug the CPU and rear fan into the motherboard if possible, and run the front fan via 4-pin molex. Most fans come with these adapters anyway.
For the 120MM fans Yate Loons are good while being easy on the wallet. If you do go with these fans, make sure you get them from Petra's, they are the only ones who deal with Yate Loon directly. There are lots of fake ones floating around.
For the 92MM: