hi,
you can use the i7-5820K with an X99 motherboard. Despite Intel's specs, it works fine with 128 GB of RAM. I tested it myself (and others have too, see youtube). It's a great processor and it won't limit your Hauptwerk setup for many many years. In my opinion, no need to go for Xeons, since they have a relatively low clock speed (you want high clock speed for Hauptwerk, especially when loading sample cache).
The i7-6700K also is a great processor, but of course is limited to 64 GB of RAM.
Good luck!