Regarding BDBS, I've seen pics of rust on aquariums after use. It's not a terribly big deal but this stuff is an industrial product and isn't geared toward keeping creatures alive. If a container of oil or cleaning fluid busted nearby it, the store may not see that as a big deal, livestock definitely will not tolerate that tho. I've seen this specific example at a hardware store where cleaning fluid was leaking into pool filter sand.
I would say my biggest tip is to be creative. I know I warned against BDBS for not being geared toward aquarium use, but a lot of stuff generally doesn't have to be to be aquarium specific to be used. Examples being if you want a sump, it doesn't have to be an aquarium. You can use a tub, or heck even a bucket. If you use a tub, make sure it can handle the water pushing against the edges. Water is HEAVY, 20 gallons is 160 pounds.
Sometimes DIY isn't the cheapest method, however DIY can often get you exactly what you need and customization options-that can often save money down the road. Filtration with a bucket filter is cheap enough, upgrading is even cheaper as often you can oversize your pump from the beginning and simply dial it up, or replace a pump which is way cheaper than getting a whole new canister filter.
More on creativity is opening your mind to what you actually need. Get a big tank, but do you go canister or sump? Sump is a fraction of the cost, but provides so much more. You can have it serve as your grow out tank for fry and plants, you can separate bit from your tank and use it as a ready to go hospital or quarantine tank. You can even use it to keep live food if you let snails run wild. A canister is a glorified mechanical filter and media holder. The sump can mechanically filter, bio filter, and so much more. Also, forgot-it can take the evap losses instead of your main tank. Hell it can even make money for you when you stick plant trimmings or fry to grow out and sell. I don't go with baffles and compartments but keep it wide open.