I can't help RE:AC units but I can say that maintaining chemistry at 20°C 24-7 isn't necessary. Last darkroom was in Chicago and temps varied widely there, and I only worried about closing the door off from the heat source in winter and possibly freezing them. I set up 3 different darkrooms in Chicago so that I could do conventional B&W as well as historical processes. Cooler is certainly better but 27 should be OK long term.
That said, even dry chemistry will go bad fast here. I shipped my darkroom from the US here and lots of chemistry went bad just sitting in a cabinet Ferric ammonium citrate turned to goo, etc etc. I'd rec a fridge or some such for paper and dry chemistry at least. As I sold off my darkroom kit here, I asked the Thai photogs buying from me what they did to keep paper stable and they said refrigerators. Keeping an AC running 24-7 in the darkroom would be darn expensive.
I have no real-world experience setting up OR using a darkroom here, but heat and humidity here are a beast for chemistry, film, and paper.