You already have a 120 at the rear so get two 140mm PWM fans at the front, £15 per fan gives you a wide range of choices. Run them from your motherboard header so they stay as quiet as possible and only ramp up when needed.
Top of the list for me would be the Noctua
NF-P14s redux-1200 PWM which are £13.99 from Overclockers.