You need to really be concerned about the following:
1.) Provisioning the equipment. I don't know how "small" a small office is, but this is going to spiral out of control quickly if you don't have an elegant way to setup handsets and make changes.
2.) Your change from circuit switched to packets. There are a lot of discussion points here, but the biggest you need to be aware of is latency is king. You might have a really slick p2p setup with OpenSWAN on 2 high bandwidth, cheap DSL or cable connections, but the jitter will kill you.
3.) How does your voice come in? If you are under contract and you have a PRI or some TDM circuit, you have to consider how you will interface that, and the cards you will need, or the SIP gateway you'll buy are not cheap.
4.) Who is going to manage the call routes, system secuity. I'm well versed with Asterisk, and you'll not find an all inclusive interface unless you go the Digium SwitchVOX route. If you don't pay close attention to security up front, you will experience toll fraud pronto.
5.) Handset support. What are you going to do for replacement parts, who is going to setup all the buttons, etc.
6.) Codecs. Some of the best are not free, i.e. G729. Just about any handset you get will support G711, but 12 bits of fidelity at 64k/sec each way (plur overheard for UDP/RTP) is not that great.
7.) Voice prompts, auto attendants, voicemail, etc.
8.) Status/BLF lights on phones. There isn't really a standardized way to do this, but SIP's Subscribe/Notify is used by some, I think Aastra.
9.) Key system habits. You won't be able to "pick up Line 2".
If I haven't scared you out of it yet, Aastra and Snom make excellent, RFC 3261 compliant handsets, Asterisk is a lot better than it used to be, and there are some alterntives you might find interesting like FreeSwitch or YXA.
Good luck.
