Those of us who have just the base game of Pillars of Eternity, who don't have the DLC, what should we do?

Should we buy the DLC, or just wait for the new version to go on a better sale?

Will the DLC be removed from the store after this sale?

If we buy the DLC, will we still have to buy the updated version?

In terms of gameplay, the base edition, the White March expansion and the free Deadfire DLC are all you need. If you want the extra goodies included in the Definitive Edition as well, it can pay to wait until it's at a price that suits you.
Basically, if all you want is the White March expansion then I don't see a reason to get the Definitive Edition.

Scratch that. Apparently owning the base game and the expansion now also gets you the goodies. If you buy the expansion and hit refresh on your account you should get them. The result for you is that you'll get nothing out of buying the Definitive Edition that you won't get by buying only the White March DLC.

Thanks for the heads up.

Maybe mail support and show that you tried to buy the game during the sale but ran into issues. They might allow you buy it at its sale price because you can proof that you did buy it during the sale.