I have a Buffalo TeraStation that I use just for a NAS to store backups on, but I did notice it has quite a few powerful features that made me think, "hey, I ~could~ use this along with a decent router to pretty much meet the needs of a SOHO"
I certainly think you ~could~ go this route, but honestly, I don't think you should. Here's why: those NAS units are pretty chill and the good ones have some kind of self-healing/recovery option ... like my TeraStation's Raid5. I've had a drive fail and it was a fairly painless but also quite LONG process to repair. Drive died, bought replacement, slammed it in... the work was simple, but it was nearly 24 hours before it was all green and fully up to speed.
Had that been my main server, I wouldn't have been too happy with that long of an outage.
However, in my situation, the NAS was just a backup, my WIn2008 server was on-line and fully available and working the whole time.
If this were an actual office, I'd have had a second win2008 server as a secondary domain controller and would have the important data set up on a DFS and that would handle file sharing. The NAS would be used as backup.
How many days could your office reasonably go with your main shared drives off-line for repair/reconstruction - even if you eventually got all your data back, it seems like lost time in a law office would be a BAD THING.