But I fear, that all my white c2d iMacs with Lion will become forced to obsolescense, if the latest version of MS Remote Desktop 8 should be refused by the Server ... (that RDP8-client version for Lion isn't even available from the AppStore anymore)
Does Mountain Lion support a newer version of the RDP client?
You can run the latest version of Windows 10 on the white iMacs. If they're virtually only used as RDP clients, does their OS matter?
Just say no to planned obsolescense.