For the OSS version installing Zimbra on CentOS would be fine, many of us here use that operating system.
The usual scenario for an A/S-A/V solution for it to process the email externally to zimbra nd forward it to us, either port 25 or via LMTP would do.
Rather that trying to implement mail collection from the multi-drop mailbox I'd suggest that just opening port 25 (plus any ports for client access) in your Firewall or Router would be adequate and getting direct mail delivery is far better. I used to run an Exchange server on a 1Mb connection without any problems. Hosting a web presence on that connection might be problematic depending on how busy your site is.