If you only have 24 users, I would look at the Small Business Server. It is an all-in-one Microsoft Server including Exchange. You can usually buy one pre-installed from the major Manufacturers and the license is a lot less expensive than a standard server. They are limited to 75 users. If you out grow it, your license can be converted to a standard server with no loss in investment.
I have one with 30 users and have no performance issues at all.
I do have a Barracuda Spam Filter appliance in front of it to stop the invalid user e-mails bombarding my domain.
If you have a good service contract and hot swap drives with RAID, you should be OK.
The only problem I had with the pre-installed software was the primary partition was not big enough to handle the data in the default configuration. I had to move the Exchange Db, and everything else to the secondary partition. If you do go that route, request a larger primary partition.
The most honest answer for you is “it depends”: Given that your site only supports 24 users, so far, then , for THAT site shold be no more than 1-2 servers: One for Your database, one for your web/email/DNS/applications.
For your VOIP, you COULD go the full-blown Unified Communications path. Since you only have 24 hosts, Cisco offers the UC500 series, which can scale up to 52 users, provide routing, switching, even wireless in one box.
For the deployment across two floors, it would work best to have a patch-panel and/or second access switch for your users to be cabled into.
Without more info, though, it’ll be hard to give an exact list of necessary equipment…