Hi
Welcome to the forums.
Yes, Zimbra works fine with CentOS - several of us use CentOS, have a look around the forums. CentOS is the same as RHEL, it isn't a derivitave, is uses the same source and is binary compatible with RHEL.
Depending on the number of users then you could get away with slightly less memory.
Do dyndns host MX records? If they do then you should be able to use it on a dynamic IP. My advice would be to get a fixed IP and 'proper' DNS records set-up.