It seems like you'd just like to fiddle with your computer. For dust u can take off the side panel and use compressed air on. a 120MM fan is probably not needed. Just check the temp on your CPU, if they don't get hot then u don't even need to bother with adding fans. If you really want to you can add a 120MM front intake fan with a fan filter And put a 80MM side intake fan with a filter.
Also the reason to have more CFM in intake is to create a postive pressure, which means its trying to bring in more air than it can put out. If it's negatively pressured(more exhaust fans)t will start sucking air from any where it can get it. Fan grills,CD roms cracks... anywhere. Simply put, air doesn't come from nowhere. So having all exhaust fans, will mean they're useless or if they're working its because they're pulling air from holes in the case. Try taking a straw putting your finger over it then sucking on it. That's what happens when you use all exhaust fans. Same thing would happen if you tried to blow on the straw of course, so you'll need to find a balance, with more preference in more intake. Note, even with filters, your case will still get some dust in it over time. Good luck.