windows-server-2012 – In-Place Upgrade from Windows Server 2012 Foundation to Standard

I don't recall the details, but Server Essentials 2012 will support up to 75 users and beyond (though at some point beyond, it's features become restricted... or at least it becomes unadvisable to push the limits of their use further).

If your clients need Exchange the choice is simple - buy Exchange and Exchange CALs or go with Hosted Exchange (most commonly, Office 365). You could also look into other solutions, but if you want to stick to being a Microsoft shop, those are the answers (and no one I know likes them either).

As I understand it, there will likely be bundles for small businesses that may include Exchange, but the discounts likely won't be anywhere near what they amounted to for 2011 (though we can hope).

Server 2012 Essentials has to be a Domain Controller, which is the first DC in the root of a new forest. It's very similar to SBS. So, no, most people wouldn't consider deploying it as a web server.