A quarantine tank is no different than a regular fish tank, other than it only houses fish temporarily before they get added into a main tank, or acts as a hospital tank when only some of the fish in a main tank are sick.
The only requirements they have are to be easy to clean and catch fish from. I try and keep a minimum number of plastic plants (obviously, since most meds will negatively affect live ones) and have a substrate that is easy to gravel vac (as opposed to sand).
Its pretty pointless to have it set up to look good though, since chances are that you're going to be pulling the plants out to get the fish out when they are healthy.
It would have the same filter as any tank of similar size. One of the only things I can think of would be that I might avoid acrylic since many methylene/malachate/acriflavine meds can stain acrylic tanks.
As for keeping fish in it , the only reason for this would be to keep it cycled. You could keep a betta, or two if you ahve a divider, to keep at least a small cycle going - but only if you have a 2 gallon (MIN) bowl for them to go in when a sick fish needs to go in the tank.
many people keep an extra, smaller filter running on their main tank and then just transfer it over to the quarantine tank when needed - this way you have an automatically cycled filter.
Or, you can keep a fishless cycle going in the tank by using ammonia to feed it.
