WK6 is probably further than I'd go back, tbh - I'd suggest just jumping in with Wrestle Kingdom 11. The storylines in NJPW are always fairly simple and are told mostly via in-ring storytelling - the big shows (like WK) have English commentary, which does a pretty good job explaining things. Promos aren't translated unfortunately, but this twitter account regularly translates things for western fans;
https://twitter.com/reasonjp
The coming month or so on NJPW World has the annual FantasticaMania shows (which are joint produced with Mexico's CMLL and always a lot of fun), plus the two New Beginning PPVs which are the next two big shows.