If you wish to have Zimbra collaboration suite installed in your server, please look for a VPS with the following specs:
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RAM: 2GB (4GB is recommended but it will work on 2GB as well)
Minimum - 32-bit OS (preferred 64bit OS)
OS: Preferred Linux (Ubuntu, CentOS)
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Also if you will be managing higher number of accounts its better to get a dedicated server or VPS with high resource.
Another thing you need to look for is a hosting company that promise nearly 100% uptime, fast customer support and may be even has live support.