I think it's a great idea to have all the stuff for a quarantine/hospital tank on hand. You never know when you'll have to treat a sick fish, or go on a spending spree at the lfs! It doesn't even have to be a glass tank - a large rubbermaid container or garbage can will do in a pinch so long as you have a filter and heater.
I have a 10 gal hospital/QT tank with a Penguin biowheel filter. There is no substrate or decorations, so it's easier to take down. When the tank isn't in use, I keep the filter cartridge and biowheel submerged and hidden in one of my planted tanks. That way, I always have an active biofilter ready to go. I guess you could do something similar with any other brand of HOB filter, or just keep a sponge filter running in one of your tanks.
When I need to quarantine some new additions, I fill the tank with water from the 58 gal tank above it using a siphon, throw together the filter, and plug in the heater. The water is of the correct pH and temp since it came from my big tank, and I don't need to add water conditioner.
After a quarantine, I simply unplug the heater, drain the tank, and put the filter media back in my planted tank. If I were to use the tank to treat a sick fish, I would disinfect everything with bleach before draining so that I wouldn't risk infecting my planted tank.
