I built a very similar thing in the US. I don't know where these fans or hepa filters are sold, but I buy a standard 20" boxfan at walmart ($16ish USD), and a 3M Filtrete AC filter in 20x20x1" size (also ~$15USD). These come in different ratings, so I buy the "allergen reducer" rating or higher.
Duct tape to the back of the box fan, done.
I would add activated carbon filter to remove even more stuff
Smart air filters has a similar idea: http://smartairfilters.com/en
Both projects use a Dylos particle counter, which is a great tool.