I live in a big city and have a big problem with dust accumulating each day in my apartment (I scrape about a fist-full of dust each day off the floors)

I also have a problem with a smell that accumulates when closing all the windows for more than half a day - sort of a faint moist cardboard smell.

I'd like to buy an air purifier and have a budget of around 3-400$ that would help reduce the dust and smell problems

What specs should I look for when buying and any brand/model suggestions are welcome

thanks in advance,

John