A QT tank doesn't need to be fancy, budget friendly is just fine.
I just use a HOB filter, titanium heater, and air-stone 24/7 in my QT tank. And several pcs of pvc pipes and elbows of various sizes so every occupant can find something they like to hide in. In the HOB filter, I use sponges for the bacteria and filter floss for mechanical filtering. It does a good job at trapping all the extra food particles (I feed heavy in QT) and is quick and easy to change out when needed.
As far as size, I like a 20G long tank for a QT tank, unless you are doing larger fish. (which I typically don't) my fish are all smaller, so the 20G size works well for me. I have it marked and always fill it to 16 gallons. Easy for dosing meds/copper.
and that size is also pretty manageable when it comes to water changes. When I QT fish, they are typically in QT for around 60 days. So I end up doing a fair amount of water changes over a couple months... Filled to 16 gallons, a 25% water change is 4 gallons. Easily managed with just a single bucket if necessary. If I end up having to use antibiotics during treatment, then I usually do a 25% water change every day for the entirety of the 7-10 day treatment... so a larger QT tank would end up being more of a pain doing all these water changes... so the 20Long seems to work well for me for QT use.
I also have several old 10G tanks kicking around from my freshwater days that are still fully useable as QT/hospital tanks in a pinch if ever needed. I always have several extra/spare heaters, air-pumps, sponge filters ready to deploy if needed.
I also like having a cheap USB camera setup to the side of my QT tank. I can keep any eye on things when I'm away....
it allows me to view the fish and more importantly, their activity/behavior when I'm not around. I can see if anybody is being bullied, being active, eating/not eating, etc.. best of all, they don't know I'm watching. Fish will often hide and act differently when you are standing in front of the tank, but the camera doesn't have this issue. So I find cameras to be a good QT tool also.