No Ravenloft anytime soon (if ever?) No problem!

[M.L. Martin]

Denizens of Dread. 3.5 reprint/update of the original monster book, Denizens of Darkness.
Ravenloft Player's Handbook. 3.5 reprint/update of the 3.0 Campaign Setting. While you could get this and convert it over, a lot of it is either available in other sources (you could just use the Vistani heritage feats in place of the half-Vistani race in here, f'rex) or doesn't necessarily fit in 4e (some of the class changes). I'd drop their Fear/Horror/Madness system for the Despair Deck, and probably replace the Power Checks system with a Corruption system derived from Concordance.

Stick with the 3.0 version--the 3.5 version is much more expensive on the secondary market, isn't really 3.5, and cuts out some material that might be useful (some of the monster stuff appears to have gone missing) while adding in some very poorly constructed and conceived material. Even if you're buying it for source material, you won't gain anything from the 3.5 update, and as I said, it's much more expensive and harder to find.
For the monster book, you're in the same situation financially--the 3.5 book is much harder to find--but both books are about the same in terms of compliance with their edition's rules . . . which is, sad to say, not always that good.

 

[The Little Raven]

Stick with the 3.0 version--the 3.5 version is much more expensive on the secondary market, isn't really 3.5, and cuts out some material that might be useful (some of the monster stuff appears to have gone missing) while adding in some very poorly constructed and conceived material. Even if you're buying it for source material, you won't gain anything from the 3.5 update, and as I said, it's much more expensive and harder to find.
For the monster book, you're in the same situation financially--the 3.5 book is much harder to find--but both books are about the same in terms of compliance with their edition's rules . . . which is, sad to say, not always that good.

My 3.0 versions fell apart and I had to get the 3.5 versions instead, but I wasn't overly impressed with most of the game design in either version.

I'd also recommend Van Richten's Guides for 3e (Walking Dead, Shadow Fey, and Mists) in combination with the 2e VR Guides.