Some of the old software before 2010 may not be able to install/run normally on x64 or Windows 10 systems. Be aware of it before purchasing it.
Skype for Business also supports the Lync client experience so that you can choose a phased upgrade approach to the new client experience for your users. For example, you might want to deploy the Lync client experience until users in your organization are fully trained in the new Skype for Business experience, or until all users are upgraded to the new server.