I have been looking into building a sealed room using portable AC myself and what I discovered in my research is you want a dual hose portable so it gets its cooling air from either outside or a different room. Other than the cost of one I'd be golden as my room was built as a cold storage room under my house and has 2 - 4" inlet/outlet pipes already in there. I used to use them for my exhaust and intakes but 4" wasn't enough so I cut a 6" hole in the stub wall on top of the concrete for the exhaust and draw my air thru filters from the larger area of the basement. It's always cool in there so have never needed AC even in the summer running 1200W of HID.
In the summer the intake air was too warm and in the winter it could be too f'n cold when it's -20 to -35c out there. The basement temp varies from about 40F - 63F so much less difference in the intake temp.
Without the AC do you run your exhaust all the time or have it on a temp/rh controller to kick in only when it's needed to reduce either of those? I have that and a speed controller on my exhaust. When it's winter the air is really dry and with say a 600W going the temp/rh may not get high enough so my exhaust may only run 5min every hour or so. I burn an alcohol lamp in there to add the CO2 the plants aren't getting from fresh air being brought in and the plants like it.
Dual hose unit or a mini-split is the way to go to save the CO2 for the plants.