A couple of comments from me intended to be fairly product neutral with one exception.
If you choose to stay in the key system environment then I will jump on the Panasonic bandwagon. I worked with them for many years before I got into the "big stuff" and still use them for my small sites in a neighboring city that it is not cost effective (yet) to network into my "big" systems.
I am looking at replacing all of my "big" systems in the next two years, and from my research, and making it as vendor agnostic as I can, I suggest in addition to the hardware platform of your choice - with all the fancy bells and whistles the sales people are going to get you all excited about (I am!), you need to look carefully at the vendors you are considering making your purchase from. How long have they been around? What is their reputation? Can you find information on their financial stability? How many factory trained techs do they have for the specific product you are buying and how fast can they get to you if needed? What kind of spare parts inventory do they keep in stock, and how far away?
Now that you have looked carefully at the vendors, look carefully at the product manufacturer and beyond just the equipment. Are they going to force you into some kind of maintenance contract, or will they let you be on a time and materials basis? If so, what level - software only, or both full hardware and software? How much are they going to push upgrades on you and how much will the upgrade process impact your business? Will they even give you the time of day if you do not keep your hardware and software up to the exact current level? How many references can you find within 150 - 200 miles of yourself of a similar size and configuration to what you will have that you can go see?
Everybody on here will be happy to tell you about their favorite brand, or vendor, or who they hate the most, and that will vary a bit depending on if they are a phone guy or an IT guy/gal, but the most important question you are going to have to ask besides how much you are going to spend and what features you want, is "OK, now that I bought this thing what do I do now"....
