Vodavi, I have some experience, not too impressed with the product. Comdial is ok, but I would not put it up there with Avaya products.
Magix is a good solid system, thing is MAC's will cost money in the long run, and the IPO customers admin can be trained to easily learn to do most MAC's themselves, or be done easily remote. The IPO cost is not far from the Partner, and the feature set is much greater. $ for $, go with the IPO for this size system.
Also, what were the objections to the IPO in the first place? Maybe I can help with them. If they have an IT person, show them the manager admin software, the voice mail pro admin software, and how easy it is to do changes, that will help sell most IT guys over a partner.
What type of business? out of office workers? There are so many features the IPO has for application to different scenarios, knowing more about the business would help. DID's would generaly cut reception hours for calls, speed communications, also adding a back door number on a DID for routing non-customer calls to an auto attendant can lift loads on reception staff as well.
You do not always get what you pay for, but you never get what you do not pay for.