I have a similar sized space and using a 400w with cool tube. same size fan w/ carbon filter. you shouldnt have any heat issues if the ventilation is set up properly.
my comments on the cool tube is that if i could do it again, i may consider another type of air cooled reflector. i have the little reflector that clips over the cool tube. I think im getting good coverage on the long side, but it doesnt give excellent coverage at the ends. That said, you could go with a 600 w safely, but if you wanted to save a few bucks, my opinion is that a 400 would work well.
good luck.