If it's just for observation, and you plan to clean it between fish, you shouldn't need much more than a tank, heater and either a HOB filter or sponge filter/air pump. I keep extra sponge filters and some matrix in my DT sump and when I want to set up a QT, I can use them and when I'm done I throw them out. And FWIW, when using an ammonia badge, I noticed the non-seeded vs seeded sponges did actually make a big difference. With seeded sponges, going 30 days without doing any water changes (makes copper easier) isn't typically a big deal. With non-seeded sponges, I found myself doing water changes every few days. Some people also use Prime or AmQuel to keep ammonia in check, or at least stretch out the time between water changes. But be careful, it (dangerously) interacts with some meds.
But you really don't need all that much to just set up a basic observational QT. Somewhere to put the water, something to keep it warm and something to keep it aerated, that's about it. I think some people just do it right in a 5 gallon bucket.
If you plan to medicate them during QT or just want meds on hand in case there's an issue, take a walk through humble fish's site. Specifically,
How to QT and
Meds to keep on hand. Copper, Metroplex and API General Cure and/or Prazipro are going to be the main ones (and required if you're going medicate during QT instead of just observe).
Also, just to add, as long as you keep an eye on your ammonia levels (or do regular 'need them or not' water changes), you don't need to worry using rock from your DT. While it would certainly help, I wouldn't want to risk contaminating it with fish disease or copper etc and not being able to put it back in without first dealing with that or throwing it out. But sponge filters are just a few dollars and Seachem Matrix cheap as well.
