I just built my first darkroom and am a newby, but will share what I know for what its worth. I purchased a $100 ac unit from Homedepot for my room, which is 8x8 with an 8 ft ceiling. Cools well here in Florida and blows a fair amount of air around. I got a used lighttight variable speed exhaust fan and louvre for a total of $80 from the LF forum. I put my Ilford fixer right under the exhaust after getting a little lightheaded in there dor several hours. Seems to have done the trick.
Regarding chemicals, I believe that developer can be flushed, fixer cannot. I use water as a stop, but I think most stop baths should not be fushed. I save used fixer in old water bottles and take them to my local landfill every month or so and hand it over to them. I wrote Photographic fixer on the bottle so these guys know what it is.
Hope this helps.