Hey Looks like you got everything going alright. I tell you what setting up before you grow is smart. I started growing and building grow room all together.
As for being on a tight budget. I hear that shit. I havent spent over 10$ on my Grow Yet! Anways I have recently built a box similar to size of yours. Mine is 4x2x6 and 6" in-line fan from Home Depot rated at 160cfm u should be able to purchase one rated at 250 CFM for 5 dollars more as for passive air intake just setup your box let the fans go. Stick a lighter outside of you 90deg PVC. If the ligher sucks in the pvc you got passive intake decently. I am still newb at growing but u can check out my Grow link is posted in Signature. Good luck
-Swisha