That's a great idea. Had never thought of that haha!
Okay small filter and small fan would be good choice, I'd personally get this mixed flow fan instead, one like this,
http://m.ebay.com.au/itm/HYDROPONICS-4-INCH-INLINE-EXHAUST-GROFAN-MIXED-FLOW-VENTILATION-DUCT-FAN-/191616554043?nav=SEARCH...