Usually by now when someone says they're going big the haterade starts flying. Obviously you're gonna fail\get busted\get robbed\get electrocuted\never try!!!
I'm going to encourage you in hopes that when you get big you remember who helped you out =p!
Soil or hydro you're gonna need an auto watering system. no way you're gonna lug that kind of weight everyday.
You'll probably want to get some real hardy strains, something pest\mold\overfert resistant, it'll make your life so much easier in the long run.
Leave yourself good sized aisles to walk down, its tempting to cram in an extra row of plants, but you'll thank yourself later if you have ample room to move around and work.
Find a hookup for your nutes- growing 500 plants you're going to go through juice like mad, get yourself a deal on it. hell find a hookup for everything! =)
Ventilation!!!!!
Measure twice- cut\pour\etc. once!
Pay lots of attention, on such a big scale things are going to happen, pests, broken\failed equipment, get on it before it gets out of hand. dilligence is the key!
Sounds like this is a closed top barn and you'll be using lights??
Think about a system like this
Electroyield Light Relay Center - Flip Flop - Timer's, Timer Boards & Relays
It lets you use a single ballast to power 2 bulbs by alternating every 12 hours, good idea not only to save having to buy half your ballasts but also eliminates the large power spike a grow of this magnitude has when day starts.
I cant really give you alot of specific plans and whatnot, just some general tips.
Good luck and happy growing
Oh also the golden rule, dont tell anyone that doesnt NEED to know. this can be the difference between bountiful harvests and a long time in jail.