My preference is a standard 20 gallon tank, which is good for one 3-4" fish for 4-6 weeks of quarantine. If you are planning on starting off with larger fish, then a 55 would be good as well.
Heater, some kind of filter for bio-filtration, like HOB power filter, good quality sponge filter, or even a home-made bio-ball filter like this:
then, like shown in that pic, several pieces of PVC pipe or plastic flowerpots for hiding places. I use 3 and 4" pvc for most fish. You can even just get a few couplers and 90's for that, instead of having to buy a full 10' length, if you don't have shorter lengths available...
Glass top or egg-crate cover is advisable to keep fish from jumping out, and a simple light is recommended, just to make it easier for you to see how they are doing. I also used a Bak-Pak skimmer/filter on mine, to help with water quality. Be sure to fully cycle the QT before adding any fish, or the ammonia cycle will stress the fish, and can even kill them...