As you noted, the current host target consists of both the hardware and software configuration. If you delete a host all of its target properties are deleted as well. We appreciate you sharing your use case with us and have taken the feedback.
A possible method to shorten the re-entering of the properties of the repurposed system could be to use the EMCLI. Using it would allow you to set all of the target properties quickly and even drive it via a file.
Not sure if this is the type of information you're looking for, but there might be Oracle Enterprise Manager Ops Center 12c that handles more of the hardware/OS layer.
It does give more hardware specific things than vanilla Enterprise Manager Cloud Control (EMCC) 12c would necessarily give you (like CPU temperature, Fan status, etc).
It does integrate with EM12c using an Infrastructure Stack Plugin.
There's also the Enterprise Manager LifeCycle Management Pack, which does do some asset management. However, that costs additional licensing ($$$) and therefore a big negative in my books; it's often so hard to quantify 'easier to manage', and man-hours spent is never an easy estimate.
If I'm interpreting your post, however, you also want to be able to put in additional meta data (things like serial#, your own support-level, etc) attached to a piece of hardware asset, correct? Not sure if that's out of the box or not.
We actually don't really need to grab additional hardware information about this system...just some way of tracking a piece of hardware so it doesn't disappear and isn't dependent upon any kind of agent. So its more a book-keeping thing than anything.
I'll look into the LifeCycle Management Pack (thanks for the pointer) but if that doesn't work, the best approach may be to continue using our own hardware management system. The good news is that it looks like we can link the OEM agents to the system via MAC address because OEM stores the MAC addresses of its host targets in the MGMT$HW_NIC table. Our current monitoring system sits on top of the hardware management system...so this might make a transition to OEM monitoring easier.
Please review the Enterprise Manager licensing guide at http://docs.oracle.com/cd/E24628_01/license.121/e24474.pdf and be aware that the MGMT$HW_NIC view is covered by additional licenses for management packs. If you have previously purchased the legacy Configuration Management Pack for Oracle Database, Configuration Management Pack for Oracle Middleware and/or the Configuration Management Pack for Non-Oracle Systems (as they were sold before EM12) this view seems to be available for use. Curiously this does not seem to be included in the list of views licensed via the (current) Database Lifecycle Management Pack which mostly incorporates those legacy packs.
For currently available management packs that view is included in the Configuration Management Pack for Applications, Application Management Pack for Fusion Applications, the Application Management Packs for Siebel and Oracle E-Business Suite, JD Edwards Enterprise One, and possibly others.
Do consult the licensing materials and your sales representative.