Yeah i just meant that if your in a room or a tent that has no airflow the plants will run out of C02 so you should leave your vents running even if temps are okay or just make C02 with sugar and yeast on the cheap. And initially a full t5 setup or 400w will cost the same but electricity-wise a t5 will be a lot cheaper to run, also bulbs cost less and temps wont be an issue, but thats all up to you.
Id recommend looking on amazon for any kind of lights or tents or fans, i think i bought my 600w for something like 300 dollars and it came with a cooltube a HPS/MH switchable ballast and a HPS and MH bulb, hangers and a timer. All that at a hydro store would have cost over 600 easy. I also got a 6" carbon filter and inline fan for 100 dollars. You dont need to pay double to get name brands like lumatek or w/e its honestly a waste of money.