What sort of ventillation fan do I need?

So I am building my grow cabinet now. The dimensions are 30" X 20" X 72". I have a vent I will be putting on the bottom of my cabinet. I hope to vent through the top section of cabinet and connect my vent fan to a carbon filter I will make myself.

What's a good economy vent fan? How many CFM do I need? Does Walmart or Target sell a model that would work? Can I hook a square bathroom fan up to a carbon filter? (My plan was to use a circular one and attach it to ducting, but at Home Depot the only circular bathroom fan the size I want is a lot more than I want to spend.)

I'll be using a 250 Watt MH with no air cooling so I need this to keep my cabinet cool and also to provide fresh air.

mine was $240,,but you can get this in a 6 inch size that goes for 150,,there made by Carrier(good name),,the model is Totaline,,sorry,,I dont have a link,,but you will find them in a n online search,,hope this helps

Keep on Growin

HoLE

Sadly I've spent almost all of my available money. I'll have to be doing this on the cheap and upgrading it later. I just set up my MH today without even putting the vent in (My poor seedlings are stretched like crazy.) Hoping it doesn't kill them before tommorow when I can put a fan in.