Subscribed...this is a very cool thread.

Instead of OSB you might consider using white styrofoam insulation from Home Depot. Their brand has an aluminum foil vapor barrier on one side that's perfect as a reflector for the flowering room. The insulation is good, too, and it comes up to 2" thick. And it's really light-weight compared to osb!

If you don't want to do a 'sealed room' you'll need to pump fresh air in, and as you note, deodorize and exhaust the air. Best to do the latter via an activated charcoal filter that exhausts through the existing bathroom skylight (you did leave it installed, right?

With three 1kw lights I'm confident you'll need a/c. One of those cheap window-mount ones from the local big box store ought to do nicely. I installed a 6500 btu a/c from Homer in my 2-car garage for working in there during the summer, and it does the job here in sunny NorCal. In a 17x7x7 insulated space that should do the trick...and it was only $70, and came with a wireless remote and digital thermostat. Larger capacity units aren't that much more money if it turns out you need more cooling with 3 lights. The a/c will even do double-duty as the dehumidifier. Just seal around it properly to prevent air and light leaks.

A setup like that will need a dedicated 50-amp circuit and proper wiring. If you need to hire an electrician, tell him you want a 50-amp 'RV circuit' added to the side of the house/garage where the trailer will sit. This won't arouse any suspicion...it will work properly...and it won't burn your house down!