You'll find widely varying opinions on "stable" CUCM releases. Some will even suggest remaining on CUCM 7.1(3b)SU2 still which is entirely defendable advise. If you choose to go with 8.0, which is also entirely defendable based on feature support needs such as VMware support, my personal advise is the latest available build if not an ES in some cases. There have been a few annoying bugs surrounding the security passphrase (CSCth87452 CSCtg62620 and CSCti18766) and Do Not Disturb with CUP (CSCti38227) that have been plaguing even the newer builds. Regardless of the build you choose, my generic suggestion is to review the bug toolkit and make your own decision on what feels most comfortable for you.
As for CUP: The latest builds of the server have been fine in my experience; the issues are in the Personal Communicator client. The CUPC 8 version is brand new code running on the CSF which is also brand new. There are a lot of bugs. I suggest the very latest build, a low tolerance before calling TAC for help, and extra testing efforts on varying PCs/video driver/software compatbilities on this front.
Lastly, Unity 8? You mean Unity Connection 8 I hope. Unity is all but dead; don't stick a new customer with it unless you have to. Unity Connection 8.5 is adding Unified Messaging support through the Single Inbox feature with Exchange. Just pay attention to the version of Exchange you are running as some of them will not officially be supported until March due to differences in WebDAV and EWS.
