What air purifier is best for keeping a small room dust-free? Also what can i use to clean a dusty room?

I am moving into my frat house in a few days and my roomate says the room is covered in thick dust, i need to buy an air purifier but i dont know which is best, i am looking to spend under $125. Also i need to know if there is any cleaning supplies or things i can use on the carpeting which is very dusty and dirty. Also the mattress in the room is dusty. Thanks for your help.