Have the basics, need a little help expert help

OK so i am going to build a 3x3x5h box and use a 400 watt HPS lighting system for flowering which comes with a 400 watt MH conversion bulb for vegetation. I will be using 2 fans for ventilation.
Now here is what i need help on...

1. What kind of wood should i use for the box itself that will be budget friendly but also sturdy enough.

2. What kind of fans should i use and what is the best place to put them (bottom, middle, top).

3. I am thinking about 4 rows of 4 for plants, does this sound good?

4. Is there anything that i didn't think of or am missing?

I appreciate all the help

I think pine wood is used for growing applications but dont quote me on that. I guess you would just want something treated though due to humidity and the possibilty of spills. The fans can be whatever you like. I would use an inline duct fan with ventillation. I would probably use a cooltube to deal with the heating issues that you'll have. To many plants for the 3x3 floor. Unless you SOG in which you will need to make another box for the flowering. With 16 plants in 9 square feet its just not gonna work out they will get too big. Thats almost a half foot per plant, thats not nearly enough.

get plywood, it will work just fine...no need to go overboard. Even if you're running a MH light the heat isn't going to escape. If anything you're going to lose heat through any cracks. But yeah, plywood sheets will cost you anywhere from $10-$20 a sheet (unless you want to cut everything from a large sheet). Buy some corner pieces and just drill that box together. Once it has its shape caulk all the corners and its not going to fall apart, trust me. The materials shouldn't cost more than $50 total.

You won't be able to flower 16 plants in that size of a box lol, much less 8.

Oh yeah, and once the box is built, spray the inside with a waterproofing agent and you won't have to worry about a thing.

Yea 16 is a little overkill i was just hopin for the most lol so i have 2 follow up questions i could use some help on. ... If i stay with the 3x3 area and 8 plants what would you suggest putting these plants into? should i put each plant in its own 9"x9" container or what would be the best way to go.

I've seen a couple people with gallon size containers, does that really make a difference?

1x2 lumber and panel wood....its really thin...makes it easy to build your room

wood wise, you can use anything really. Closed grain wood is better but there no need to spend more when plywood will work just fine