I second all of @pilow points. We have been running a lot of Dell R630's with Intel CPU, Intel NIC, PERC H730i raid-cards and it has been flawless. Now we're using Dell R660's, but with newer spec's. Tried Broadcom NIC's once, it worked, but had some weird performance issues from time to time. Go with shared storage, NFS if possible (thin provisioning) and avoid XOSAN/XOSTOR, I don't think its battle-tested quite yet.

@giejo In addition Vates is willing to make fixes and/or additions to their virtualisation stack. Though for LTS releases its dedicated to stability and security after release, so major new features will only be added in LTS before a certain stage in its development. Though are regularly making fixes and additions to Xen Orchestra. That maintenance mode for SR in Xen Orchestra was result of my idea, from brainstorming what large deployments might require. Also didn't mention but Xen Orchestra also supports the use of Web hooks and Vates has a public API actively in development. If you find that the public API is missing feature(s) which would be of help to you and/or other users just let them know so it can be worked on for the next update.