I'd recommend that you consider TMS15.6.1. otherwise you may need to upgrade again in the near future, so, if you're going to the effort of upgrading/replacing your current server, you should at least think about the next steps too.
If you check out the TMS Install and Upgrade Guides, you will see that the SQL Server Requirements have changed, as well as the minimum version of Windows Server, so it's recommended that you build a completely new server with a current/supported Windows and SQL version.
All the steps for moving from one server to another are documented in the guides (ie if you are going to TMS15.6.1, it is documented from Page 54 onwards).
Wayne
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