The DRM on the retail PC version of D2:ED is just a disk check...

I have not seen any mention of compatibility between disk and download versions of the game and add-on. Presumably Steam will list the requirements when they add an entry for FoV, or Larian may comment before then.

Saved games are very likely compatible between disk and download versions. Before the 1.03 patch several people were able to get around a problem with the game locking up on the new game window by using a save from someone else, though they were not specifying whether they had the disk or download versions.
I'm not sure about compatibility with the Dragon Knight Saga, though. Since the remastered version includes a fair amount of changes (not a lot of details yet), that might cause issues with D2:ED saves. From the topic Another status update, about DKS:
Originally Posted by Lar_q
You can take that "remastered heavily" part seriously as we've literally redesigned almost all dragon parts, especially the flying fortresses, drastically changed loot/enemies and retouched quite some artwork. The net result is a much more streamlined experience that is a lot more fun (we think wink )