For WAS it depends on whether you want a carrier fleet or a battleship fleet.
I bought mostly singles and a starter and I think two boosters.
I bought an Enterprise and two Devastators and two Dauntless and two Wildcats. I picked up 2 Atlantas.
I bought an Akagi and similar aircraft as for US. Everything else is pretty much what I got out of the starter and two boosters I picked up.
I have decent if not good 100 pt fleets for US and Japan and okay 200 point fleets. It is plenty for casual play with OOB rules.
I suggest grabbing a Starter set and two boosters and then fill in your holes. WAS doesn’t really have an I win piece. Maybe the Iowa or Yamato but enough planes can handle 'em. And the Atlantas. You should certainly have an Atlanta which I think is a great utility ship. One in every US fleet.