Grow Box Size?

I've seen some people have all three compartments stacked on each other.
 
If You plan on doing this make it sturdy, it could be a great fire hazard if a level falls and breaks bulbs, or if the bulbs come into contact with water.
 
I would go for at least a 3x3xwhatever hieght You would like.
 
But all this depends on how big You want Your plants to be.

 

 
 
When starting a grow op you need to answer one of the following questions so you can build everything else around it.
 
* how much bud do you need per month? 
* how many kWh can you afford to use per month? 
* how much room do you have to dedicate to growing? 

 

That's pretty cheap man, I prefer dimmable ballasts personally. They come in handy if you suddenly find yourself struggling with temperature so you can turn it down a notch. 
 
With a 600w you could do with a minimum of 3'x3' really. The vegging area could be less, 250w or 400w would be plenty.
 
If you are vegging and flowering separately you can harvest every 2 months. With a 600w you can harvest 15-18oz per harvest so you will get way more than your goal. You could achieve your goal flowering with a 400w.
 
I have my 600w in a 4x4' tent and if you have the room for it, it's a good size. I have a 350w LED for vegging. it only draws 270 watts so quite a mild light but seems to work well. before this I was using a 250w MH which worked well, and before that I used a number of 125w CFLs, and they worked well too.
 
I think you would be better off having a flower area, and a veg/clone area. Clones don't take up much room so there's usually no need for them to have a dedicated area. Setting up 2 areas is easier than 3.

 

For a perpetual harvest you describe, you only really need 2 spaces(veg/clone - flower). At any given time you would probably have 4 plants in flower(if 55-60 day flower time). If using hps to flower, it has been my experience(and others), that any closer than 2' from the canopy lowers taste and potency big time comparatively(especially last 2 wks.!). I like flowering sometimes using MH, even though MH will produce a smaller harvest than HPS. I find potency, taste, legs, quality in general is higher, especially with sativa doms(I'll sacrifice quantity for quality anytime!). I would recommend at least one square foot of space for each flowering plant for optimum quality and yield, then figure what size plant to go with (2ft, 3ft etc..), plus headroom for light and fixture. Keep it Green! Peace!

 

If you want no smell at all then you will need a filter in both areas. But the veg filter won't need to be renewed very often. You could rig it up so the air comes in to the veg area, then in to the flower area and then through the filter and out. 
 
I recommend using an air cooled hood with separate fan. This won't need a filter as no smelly air will get in to it and it removes most of the hot air before it even gets to the plants. I have mine set up like that and the difference between inside the tent and outside is just 2.5℃
 
I have a 4" fan and filter venting my 600w flower tent and a 5" fan venting the air cooled hood. I would think if you don't use an air cooled hood you could get away with a 6" fan. But I would probably get an 8" to be sure. What size will the flower area be? 

 

 
Tents do make for a lot less work. I have a flower tent and veg tent, they cost me £50 each and they are good to go, reflective walls, light proof, and vent holes for fans already sewn in