You should only need to buy Shadowbringers as it includes previous expansions, but I'm fairly certain that you wouldn't be able to access your character on the non-upgraded account (which would be PC for you) until release (or early access if you preorder) due to its level and possibly the location that it is left at. You'd still be able to play on playstation until release, though if you plan on returning to ps4 afterwards you'd have to get the expansion for it as well.