I guess we could go around and around with this. Is this server already being used for other services besides Zimbra? If this is the case you might have some other issues on your hands. If this is not the case then I still don't see why you wouldn't start from scratch.
But I guess we'll put that aside. Of the options you've outlined I'd go with the VM. Don't try running the RedHat binaries on Fedora. I've never even thought of trying it, and maybe someone will jump in and tell me I'm wrong, but I think you're just asking for trouble with this option. I've also never tried compiling Zimbra from source but it definitely is going to be more complex than installing from binaries.
As far as your question on the resolv.conf file, I'm not sure the search line is necessary, but it probably would be best to add it. You would want to add a line such as "search domain.com". Basically it allows you to access systems on your LAN using just the host name instead of the FQDN. Not sure where, or if, this would be necessary with Zimbra, but it is kind of standard practice.  | 
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