1. Make sure you create a filter box at the end of your duct, so that all the air entering your attic will be odor free. Ducting and light fixtures have small cracks, which is a security risk.
2. The air conditioners that you own, do they have a separate intake and exhaust duct? If they do not, then they are a security risk. If they pull air from the room and exhaust it outside, you will exhaust all of your CO2 and odors. So this is a very important question. If they are window unit types, you must make sure they have the option of recirculating the room air.
3. Buying another air conditioner....the answer is dependent on your answer to number 2 above. If your air conditioner doesn't expel odors and CO2, then I would just by another one.
4. Sizing of your AC...If you air cool your lights, you'll have to run your room at night. If you're testing out these types of AC limits, then make sure you buy one that you can return, because I feel you are shooting too low. For 6 lights, I would normally use 36k btu, 6k per light...that is for a sealed environment. You might be able to cut that in half, by air cooling your lights, but you'll have to test it out to know for sure. Just make sure you can return the unit if it's too small.
5. If you have $2000 to throw towards AC...I would go out and get a 3 ton mini split, then you could run a completely sealed room and you will save money by not having to build the filter box, buy the filter, fan, ducting and labor. To air cool 6 lights, you're already spending like $1k. Fan is like 300, filter, 300, box 100, timer 20, ducting 20, electricity to run the fan, plus the labor...so, instead of doing that, why not just put that $1000 towards a larger AC? Avoiding incriminating odors is priceless in itself. If you insist on air cooling your lights, then at least make sure you build the filter box, or push your air through with a filter on the end.
Don't fuck around with being cheap on Odor control, that is one of the most important aspects of our business. If you are running 6 hoods, safety IS important.