When I prepared a Curse of Strahd adventure, I took a look at the original Ravenloft module, as well as the 3e Expedition to Castle Ravenloft expanded module. I had no interest in the supplemental material that expanded it into a full on campaign setting; nothing against it, just I wanted to stick to a simplified module. My goal was to find the simplest story I could come up with as a core, and decide what to expand upon after that. If you're going for a full on setting to write your own material, my resources aren't very helpful.
My main reason for liking 3e conversions of older modules was a better attempt at appropriate CR and party balance, tactical encounters if using map and minis, plus expanded story. The 3.5 Expedition book has a lot that I could use for inspiration, using all three sources to see what I really liked core to the base story, what I felt was lacking in the original, and what I felt was expanding way too much in updates. It definitely goes in a different direction for side story than Curse of Strahd, and I think the inclusion of so many monsters from cross-promoted-at-the-time Libris Mortis supplement makes it a bit harder to convert over to 5e.
If you're going in to Castle Ravenloft itself, I'm quite disappointed with the isometric view of the castle provided in the books, especially if I intend to use a gridded mat and minis. These
fan-made simple maps were a great help to me, and I believe the associated numbers line up with the original module as well as Curse of Strahd. I have no experience with this, but the DM's Guild has a product to
print and run the full castle.
The artist for all the maps printed in Curse of Strahd (overview of the whole land of Barovia, villages, locations) also has
high-res digital versions available to purchase from his website.