Here's a few things to consider.
1. You would need to install Patch Manager 2.0RC if you go with Server 2012R2 as this OS isn't supported on version 1.85.490.
2. The WSUS console version on 2012 will not match the WSUS version on the 2008R2 server. One workaround for this is to install an automation server on the WSUS server, and create an automation server routing rule to push publishing actions to the WSUS server. Here's an article on this scenario. http://thwack.solarwinds.com/community/solarwinds-community/geek-speak_tht/blog/2013/01/29/managing-wsus-on-windows-server-2012-with-patch-manager-v185