Network... look into cisco and HP for starters. We just put in new HP core layer 7 switche, and it will do VOIP as well as power over ethernet. You will need a firewall, and probably a few servers. Think SECURITY!! it is a law firm... information is most treasured there. A lawfirm of 25 will also house an internal research staff, they will have specific database needs... SQL most likely...
VOIP is actually pretty easy at this point... Once your network hardware supports it... it is just a matter of ensureing you have the right phone equipment (read phones and voip server). You will still want standard phone lines coming into the building and split off... I would say 4 as well. 3 for regular use, and one fax.
Desktops are easy, most would be laptops I would imagine.
Server wise, I would think 3 or 4 is enough to do everything you would want including local email systems and such. Work in a budget for maintenance which wouldn't fall under the 100K that would allow them to call someone for support.
Again, do not leave out security measures...
that is a 50000 ft top down view. I would get basics down, and then step through the day of an attorney to see whatever needs they would have. Software use and such. Don't forget a voice mail system as well... often times, it is not included with regular voip phone service.