A small Exchange server really isn't too hard if the client doesn't want anything too fancy.
It can get more complex if you need to do multiple domains and such, but I would doubt you'd have that with only 12 mailboxes.
The key to Exchange is building it to where you can recover it well when it gets corrupted. This means a good backup plan and having you files structured right.
I usually make sure the server has a minimum of double the necessary space needed for the message stores, and triple if I don't have any other network space available to move stuff to. Having the space available makes it much easier to make multiple copies of the databases if you have to work on them and being able to copy the whole set once or twice instead of being crammed for space.
I actually just got done with a 16 hour Exchange recovery a couple of hours ago, and it probably would have only taken 10 if I had more space to work on, but since I didn't, I had to move files back and forth between two drives one at a time because there wasn't enough room for a whole set of backups on either of them.
I'd say get a book and go for it; Just remember that e-mail is very important to companies, and losing it really isn't an option.
Find someone local to where you are to consult with for a couple of hours about how it's setup and to show you where stuff is, and then keep their number around in case you need someone experienced to help with a recovery. If it's not done right, a lot of destruction can be caused by the recovery tools.
Day to day maintenance though, you should be able to figure out by playing with it (in the case of adding mailboxes, e-mail addresses, stuff like that) and research (in the case of backups, good practices, etc.)
If you have any specific questions about the system you're going to be working on, drop me a PM.