New grow room, please help with lighting amount!!!

So, I had a thread started 4 days ago with a room I had originally planned on using to grow but I decided im gonna restart small rather than go balls to the wall. Let me know what you think....

The room is 12.5' long and 7.5' wide with a 7.5' tall ceiling. There are two windows (its a shed) which might be very useful for ventilation in and out. Considering my dimensions I was planning on 8-10 plants... i'll probably more comfortably be able to fit 8 so lets go with that. How much lighting should I install?? Im fairly set on 600 watt HPS for obvious reasons regarding lumens etc. Do you think 3 would be sufficient or should I go 4 or even 5? At which point would I just be wasting electricity?

Also another concern is the shed height. Im going to probably be growing in 3-5 gallon buckets with holes drilled in bottom and I understand the lights will take up space overhead so I cant grow some monstrously behemoth plants. I was thinking 2.5-3 feet before flowering, would that leave me enough height to space lights from plants, plus allow for any additional growth while flowering??

Any suggestions are VERY welcomed... also is there a preferred amount of watts per plant? And if so what is it?
+rep to all who give helpful info!!!

please? help?

Is your budget high or low?

get 1000 watters. Switchable ballasts. AC window unit. Aircooled hoods.

why 1000's over 600's? I have a good budget, a few grand to throw around. Dont really wanna do AC tho, if I provide enough air flow I should be good. How much is a small AC unit?? AC units r loud... I donno.... am planning to cool lights via inline fans though

x3 600 would be fine if you get some good hoods for em. You should look into AC tho... Its good to keep a room like that at about 75-80.. 85 tops my ac also works as a dehumidifier as well.. 2 birds one stone weeh.