Hi Tom
If you can give more information about the current system will help to assist you.
For example the number of phones and type of phones required. I would also suggest checking if there is UCSS for the existing Call manager as well.
See below for party numbers and servers that you might consider for the solution.
Unified communication Manager
1.0 MCS7835I2-K9-CMC2 Unified CM 7.1 7835-I2 Appliance, 0 Seats ( consider PUB/SUB) so 2 X servers
2.0 License Unified CM 7.1 7825 Appliance 1000 seats( consider PUB/SUB) so 2 X Licence)
3.0 Device License Unit (DLU) depending on the type and number of phones/Mobility enabled users you have.
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps556/ps6164/prod_qas0900aecd805009fc_ps556_Products_Q_and_A_Item.html
Unity Connection
1.0 Cisco Unity Connection 7.1 Appliance - MCS7825I4-K9-UCB1 x
2.0 License for users example 100 users, 24 ports UNITYCN7-100USR ( this depends on number of users)
Cisco Unified Presence
1.0 MCS 7825 PreLoaded Servers IBM - MCS7825I3-K9-CUP7
2.0 Cisco Unified Presence 7.0 Application Software - SW-CUP7.0-K9
3.0 Cisco Unified Presence Server User Licenses - CUP-Server-LIC
Note: - As I mentioned it before I would suggest checking if there is a license (UCSS) for the existing servers so that you can take a migration path and the part numbers your require will be slightly different.
I hope this Helps.
Mohammed