I was thinking of keeping it simple and getting two of those. One for the cpu, one for the hdd intake.
Yes, a PWM fan on the CPU cooler running at low speed at idle would make sense. Usually it would be better to pair it with a PWM exhaust fan. So you could consider moving the existing exhaust fan to the intake position where it can run at a lowish speed just to keep HD temperatures in check. In terms of fans, the Scythe adjustable PWM fan SY1225SL12HPVC costs around £10 from sources such as QuietPC
. The adjustable feature should give a greater control over idle speeds than most motherboards allow, typically at the lowest setting around 400 rpm.
Would I need a pwm splitter?
For a two fan PWM chain of CPU plus exhaust a simple PWM Y cable costing about £3 such as this one
will be OK.