Look on Craigslist for used fish tanks I always see people selling it, some are priced very reasonable. Just make sure there's no cracks or worn out silicone on the seals.
I think it's suitable for betta's to be in a low profile tank versus height cause they have to swim up to the surface to breathe. A 20 gallon long tank would be great but very oversized for the size of the fish. Just a thought.
You have hard water on the alkaline side, if your betta likes that type of water condition should be ok. Water plants on the other hand like neutral ph. so you might want to do some research on the plant background. Good choice on plant selection, I know they don't require a lot of light to...