I had my darkrm in one of the 5room flat. I am able to place cardboard on the windows to seal the light and remove them when i have to. And it's comfortable to have an air-condition and a radio so it wouldn't be so boring. The heat emitted from the enlarger lamp is very hot so air-condition helps. Safety light is a 220V philips red bulb. Just clip it to the window grill.
The big enlarger(secondhand) has to be place on a solid table. My case it's a small computer table and can be wheel around easily. Placing the 3 trays on a table or floor and after fixing i leave the room carry the print on a 4th tray and wash them at the toilet. As i walk in the out, ventilation is not necessary. Furthermore, my room is big so not so stuffy. Anyway the strong smell comes from the fixer. i think... I check my prints in the toilet, fluorescent light, some say it's not the right kind of light?? Anyway the prints are whiter when wet and darken slighty when it drys up, maybe its the light? Finally after wash, i squeeze off the water with a rubber scraper (for windows) and place the wet prints on newspapers and let it dry up naturally.
After printing, just cover-up the enlarger with a big plastic cover to prevent dust and tuck in one corner and remove the window cardboards. And it's back to a normal room again. I open all windows and door to air away the smell. Fortunately, my room is very windy. And my flooring are tiles so i'm not worried about staining them, not sure about marble or paraquet.
My toilet, no doubt is quite big and nice for a wet area and it's convenient whenever i need a leak, but it's still not so idea. And since the area is small i believe ventilation is a must. And it's a hassle moving the heavy enlarger (big meopta, 6X9) and table in and out....phew..
They most important thing is DUST free. Try to keep as less fabric/cotton items away from the darkroom as possible. And minimise the air movement when you are printing. It's really annoying to find dust on your negatives or masking glass.