Voip and Asterisk newbie question

A co-worker has asked me to help him on his Voip business that he wants to start. I show him how I can configure 2 grandstream devices with a sip server that can talk to each other. He was planning to use 2 voip modules to connect directly to a Nortel phone extension and then get a dial tone to dial back out using VOIP phone service from our phone company. He wanted to use the Nortel system, since he knew it very well and that he can have accounting done on each extension so he can bill them. My other friend told me that I wouldn’t need 2 VOIP devices, only 1 and that I could use a PBX system other than a Nortel system. So that brings me here. Can Asterisk work for me?