Quarantine Tank

I can tell you what I do 

I have a 25 litre tank, 45cm long. No light in the hood, just a plastic lid. With no real plants you don't need a light, and some medications say to turn the light off - a quarantine tank doubles up as a hospital tank. Mine is not a make. Just a plain 25 litre tank, even the sealant in the corners looks basic. I got it from a local shop.
Yes you can use plastic as long as you position the heater so it doesn't melt the tank.

I use plastic plants and 4 inch lengths of drainpipe as caves. Choose carefully, some types of pipe sink, others float. Mine are the stuff that takes the rainwater from the roof rather than water from the sink. If the worst happens and the fish in there die of some disease, plastic is easiest to sterlise with bleach.

Substrate. The main reason for not for not using one is that you can see the fish poo easier. Poo is often a good indicator that something is wrong - long stringy poo is indicative of intestinal parasites and internal bacteria infections. If you really do want to use something, a thin layer of sand is better than gravel as you can see the poo better.

I have an assortment of filters. I have a few Eheim Aquaballs (bought cheap off ebay) My main filters are an aquaball and a biopower so it's easy to swap media. I also have 2 rather old (and obsolete) Hagen Elite mini filters and in my betta's tank, a filter that would be suitable - and aqua-internal 50. The problem with that is that it's Maidenhead Aquatics own brand so that's the only place you can get them from. Both that and the elite minis have just a plain sponge (the aqua-internal also has carbon in a hollow in the sponge but I emptied that out!). The simplest thing is to forget about the small filter's own media, just take some of the main filter's media and squash it in when it's needed.
Don't forget you can leave the whole small filter running inside the main tank when the QT isn't in use, then just use the small filter when needed, no messing about with bits of media.
Be careful of the media inside a lot of small filters if you decide to use its media. They often have cartridges containing carbon and zeolite. These adsorb/absorb medications so can't be used in hospital tanks or QTs if the fish turn out to be sick.

It was much better than the stingray  .This. Small, adjustable flow, perfect for a betta's tank. They've stopped making it  and mine are just about falling apart. The replacement is one of those with nothing but carbon and zeolite inside 

Yes, a couple of mm of sand would be fine. Just so you can't see the bottom - and I suppose the fish won't be able to see their reflections if that's what happens under water. I may have studied chemistry but not physics