The most important consideration is how tried the solution is. We are a Cisco reseller, a PBX reseller, and a network/LAN/WAN reseller. I will not try to sell you on anything. I will tell you that you need to be sure of the technology you need to purchase. Cisco, 3COM, and Altigen all have products that perform IP telephony bu they are all very young phone system companies. Some of the traditional featus you enjoy on your NEC PBX will take years for these companies to learn and duplicate. Meanwhile, even large companies like Avaya, NEC, Nortel, Toshiba, et. al have very young IP products that have not been tested or implemented behind multiple configurations. Between your WAN config, your IP config, and your LAN config, the most important thing you can do is find a vendor who has RELEASED and TRIED products. You also need to make sure your telecom vendor has a very proven set of CTI applications and a strong ACD solution. You will need to deal more with deliverability than marketability.
My thoughts,
Kevin