Yeah... I hate having to do this, but...
You are going to fail. One, 600hps won't be enough. You might get by with one 1000w or 2 400ws (heat may be an problem though). Light issues aside, your time frame is far to small. You might be able to do it if you acquire some mature clones from a quick blooming strain. Finally, 15-20 plants is going to be tight in that space, but not impossible (not impossible for an experienced grower maybe).
Anyway, don't get discouraged by a pessimist like me. I have been proven wrong before. All that I'm saying is that don't be too ambitious, you will make mistakes that will cost time, remember to factor that in.