For one of the projects I'm working on, the site currently has a workgroup of around 15 PCs (XP/Vista/7- these are employees' own computers) and one file server running Server 2008.
They use externally-hosted email (accessed in Outlook via IMAP) but would like an Exchange server in order to centralize Outlook contacts/calendar info, get mobile syncing via Exchange ActiveSync, and perhaps add web access via OWA.
Can Exchange be set up in a workgroup, or if I set up a domain with the new Exchange server (or SBS, whic might be a more financially viable route), or the current file server, as a PDC, would that work (given that most of the clients are running Home Premium OSes)?
The only constraint is that configuration changes on the clients should be limited to Outlook if possible (e.g. I can't have them all switch to Business edition OSes).
First, to answer the question. I believe that Exchange must be in an AD domain. However, I believe that it can be it's own "standalone" domain, which is essentially the same thing. The PC on which Outlook is installed does not have to be in the domain, so the Home Premium PCs should be OK.
Now, some additional thoughts.
1- The things you want to do (Active Sync, common contacts and calendars, OWA) can all be done on a hosted Exchange environment. Two good providers are MainStreet and SherWeb; there are tons more. Almost certainly it is better financially than rolling your own.
2- SBS is also a good solution .. I recommend it over building a single server domain and Exchange installation.
3- There are other EXCELLENT reasons for installing an AD domain if you aren't going to outsource. See the questions here, here, here, and here.
4- I would not be OK with employee-controlled PCs direct access to the server unless I had a good handle on the anti-virus and security setup on those PCs. In networks I manage, PCs not provided by me get very limited access to the infrastructure through a firewall.
Exchange doesn't support running in a workgroup. It has to be installed within a domain. Without the clients computers connected to the domain (as home OSs don't support connecting to a domain) the users will need to authenticate against the domain every time Outlook opens.
Given the size of your organisation and requirements, consider Microsoft's cloud Exchange offering:
http://www.microsoft.com/online/exchange-hosted-services.mspx
It'll meet your requirements. Your scenario appears to match up perfectly with the current 'ideal case' for cloud-hosted email providers.
You could also look at google's professional mail hosting option. Again you'll get your calendaring, address book, device sync, spam filtering, IMAP connectivity, all as part of the package.
Managment will love the low up-front cost (basically just man hours to migrate) and the monthly per-user spend.
The last company I worked for used Rackspace's hosted Exchange product, which worked quite well. For a user base your size, it should have a much lower total cost for similar benefit, while meeting your support requirements.
Single Server as a DC + Exchange Standard will do what you need (that will likely be cheaper then SBS). Clients will connect and just need to be prompted for username/password each time. Though if they setup their local account with the same name/password as the server, it should just go pass though. It does for a file share, can't remember if Outlook is the same.
Hosted will also do what you need.
Before going with either option, do a true cost compare of the 2 options. Call an MS reseller and get an real quote for Exchange, add in your time to setup, time per month to take care of it, etc. Then do the same for a hosted Exchange, that will also take some of your time as well, unless your users can call the hosting account for support directly, in which case it will some what less.
Prorate it out over 2 years to give you a 2 year TCO.