Help me getting started

[Perodigy]

Hi,

I'm currently thinking about getting back into pokemon from scratch.
That means I wanna go take it seriously so for me that means getting all legit available legendary pokemon shiny, getting, all shiny starters, getting competitive pokemon etc.
However in order to do that I'll obviously need to get alot of games.
So my question to you guys is which pokemon games do I really need in order to do the things listed above.
(I'm currently planning in only getting 1 3ds and when I get completed a few games get a 2nd)

Thanks in advance