To Ravenloft or not to Ravenloft? That is the question...

Hi, I'm finally contemplating delving into DMing but want a different setting to our usual Forgotten Realms adventures.

Ravenloft looks like a great candidate but, with the advent of 5.5e, I don't want to be investing in unessential purchases. 

So, simply put, does the new Ravenloft: The Horrors Within replace the earlier releases, Van Richten's Guide to Ravenloft and The Curse of Strahd?

I believe so. At least in 5.5e.

Exactly how much it replaces remains to be seen. The new ebeeron book didn’t exactly replace the old one, it basically updated it, but the older one was still useful. We won’t really know how much of this book is updating old things vs adding new content until June 2 when the early access comes out. 

The biggest thing with Ravenloft is that you have to understand that ~to be blunt~ you are going to be inverting the nominal D&D expierience (IE empowering the player) so that they feel small, weak and vulnerable because at it's core ravenloft is a horror setting. This... can and will cause problems for some players as they aren't going to get the straight forward good versus evil campaign where they beat the BBEG into submission; in many cases defeating the BBEG just makes everything worse for the domain just in new and terrible ways and that's assuming that the players are even allowed by the dark powers to pull off this kind of feat.

So at the end of the day... I say it's a case of "maybe?"