Under <SEP 12.1 CD 1>:\SEPM\AdditionalPackages you'll find either a legacy SEP client v11.0.6300 (SEP 12.1.671) or a legacy SEP client v11.0.7000 (SEP 12.1.1000). SEP 11.0.1000 was completely outdated for years when SEP 12.1 emerged on the market.
For SEP version numbers, see this document:
https://www-secure.symantec.com/connect/articles/what-are-symantec-endpoint-protection-sep-versions-released-officialy
2. How to convert manage clients to unmanage clients?
As the others say, it's perfectly fine to protect W2000 servers with SEP 11 in a SEP 12.1 environment. SEP 11 clients can be managed by SEP 12.1 SEPMs. Therefore, I don't know why you want to make an unmanaged client.
However, to convert an managed to an unmanaged client, update the client with the sylink.xml file from the SEP installation CD (folder SEP). See this document:
How to convert Symantec Endpoint Protection (SEP) clients from managed to unmanaged without uninstalling and reinstalling
http://www.symantec.com/docs/TECH104010
and in this case, 2000 clients will get the update directly from symantec server?
Can you elaborate more precisely? Unmanaged clients don't get any updates (or policies) from their SEPM, because they don't have any. They are stand-alone.