first off, good call
nice to see a reefer taking this sort of precaution...
small 15 to 20 gallon tank is all you need... a little rock for bio filtration... personally, i would use the sponge jus to catch detritus and rinse it everyday, being sure not to let anything build up in it, basically using it as a "filter sock"... definately gonna want a small heater as the small volume of water with the external hang on filter will not hold temp very well at all, and you want it stable, and controllable... I always like alot of flow in my QT's personally, as it helps make up for the lack of space, and keep the fishing moving, and oxygenated, as well as helping them use their muslces a bit, as they have most likely been in restricted conditions since they were pulled from the ocean... however you should QT captive bred fish as well...i always QT with some sort of shelter, a couple pcs of PVC are all you need for this... i always use copper based meds, as well as Prazi, or something along those lines... also if your gonna do hypo, be sure to get your salinity down to 1.010 for it work properly, however if your gonna attempt this, convert your salinity slowly over at least a few days, and do some reading up on it first, and get some others to advise you... myself, I usually jus exchange cup or two of QT saltwater, with RO water, and bring it down that way, but again, very slowly... dont use display water from water changes as your QT water change, as it may not be clean, and even if you took the necessary steps to have the cleanest display, its jus not a good idea...
be sure to feed well, as you wana build up their immune systems as well, it is wrong to not feed them properly and plentiful in QT imo, as some will say different due to water quality issues... I say this is where your husbandry and diligence come in, and you have to change a water every few days at least, i do a couple gallons everyday when i run a 15+ gallon QT for my fish, which will be going up again soon as i plan to add 2 more small fish to my display... you really wana make sure you feed your fish the best foods possible imo... and try to get some vitamans for your mysis, which i recommend vitaman enriched mysis, spectrum small pellets, Nori for herbivores, which can also be vitman enriched... be sure to QT your fish for the proper lengths of time, or its all for not... A few weeks minumum for QT of any fish... preferably 28 days...
Your doing a very smart thing imo... be patient, cautious and take preventive measures, it will save you alot of headache later on... and if your like me, a little heartache too... i get attached to my fish and corals with the time i put in...
Good Luck