Bettas will benefit from good water stats like any other fish - including a good water change regimen. One aspect of this that is different for a betta however is that they like still water - they don't like a strong current (their evolutionary background supposedly being that they are used to living in "their own puddle.") (I'm not a betta expert by any means, so those who have raised a lot of them might want to modify or add to this picture!)
Not sure what you mean by "a plant dechlorinated water," but I'm assuming maybe you hope to use plants to balance the environment for the betta. A heavily planted small tank with a single betta probably would indeed approach being a balanced situation - particularly with frequent enough water changes. However the usual problems would still apply: if you suffered a problem with the plants and had a significant die-off, the filter might not have enough bacteria to process the resulting ammonia and nitrite. If you are a beginner, this puts a bigger burden on you to know more about keeping the plants healthy.
Anyway, just some thoughts, ~~waterdrop~~