For a very small office, a Cisco 805 router will run a T1 fine. You need an external CSU/DSU, however. This works best in a WAN situation where you're connecting to frame relay or the a leased line. I used these a couple of times for some of my smaller customer WAN sites and they seemed to do the job just fine.
If you're connecting to the Internet with your T1, I'd go with something more powerful and better-designed for security and connectivity.
That being said, if you've got the cash, go for the 1800-series. It's a much better and faster box. You'll pay about another $500 more for a 1841 and a T1 WIC, but it's probably worth it.
- G