You can start with a 7815/16 which can handle up to 300 phones but only two servers in the cluster. I would also recommend going to the Callmanager Business edition which has callmanager and unity connection built into one server. Right now there is no redundancy from a server standpoint, but this is in the roadmap. Note that Cm 6.0 has not been out there for long, so you may want to watch out on what kind of bugs you will run into before buying one of these.
I would go ahead with a 7825 for a min. This can handle up to 1000 devices.
Use callmanager capacity tool for sizing.
http://www.cisco.com/cgi-bin/CT/CCMCT/ct.cgi
You will need ur CCO login.
HTH
Sankar
PS: please remember to rate posts!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus