Checking room temps a head of time is a good idea. What are your room cooling options, AC, low out door temps? I think that you can get away with a 400w in there, but I also think that it will be some work. Most don't figure how much work it can be once the heat stoker attacks your cab, so just be prepared to adjust what ever you come up with, meaning that it will probably run hotter than you want at first.
As far as CFM I have found that it is more the brand of fan that determines the CFM that you need to use for example my 172 vortex puts my 260 squirrel to shame.
Hope that you can invest in a good fan it can make all of the difference.
Adieu