Congrats on getting your own space up and going. I have several recommendations. First of all, you are going to need to make your space smaller--say something around 5 x 6 or smaller. Two 600W and 2 400W are only good for about 56 sq ft (at the most). We figure lighting needs by lumens per sq ft with 5000 lumens per ft being about the minimum you want to run when flowering. Vegging plants do well with a minimum of 3000 lumens per sq ft. A 12 x 12 space would need at least 5 1000W HPS to be adequately lit.
Next, I would recommend air cooled hoods. If you get your space the size it should be you are going to be generating some heat. It is very very hard to keep non air-cooled reflectors cool enough.
What type of inline fan do you have? And what are your ventilation plans? Even if you have a centrifuge fan and air cooled hoods, I don't think that a 6" fan will be able to keep the space cool. Even if the room is 56 sq ft, a 450 cfm fan will only exchange the air once a minute. I use a 6" Vortex in a space that is 3' x 6.5' and it is not too much fan.
I think that whether you go with the larger buckets or not will depend on how long you veg them and how large they get.
Whether or not you will need a humidifier will depend on the RH where you live.