Hi,
I work in IP comms and have deployed many systems from small business to enterprise.
We focus on systems from Cisco Systems.
The Cisco UC500 product would be a good fit for your needs.
Cisco has a reputation as being expensive but, by choosing the IP phone handsets carefully, a system can work out pretty reasonably.
When choosing a replacement system make sure that you understand all the costs – e.g. a network upgrade to support Power over Ethernet may be needed. Also think if there are any features that your old system did not offer that you would benefit from – for example, Single Number Reach which extends desk calls out to mobiles and inbound screen pops of caller details.
If you would like more details of the Cisco solution please let me know.