There is more than one "VoIP" - each can be considered.
Also understand that VoIP is Voice over Internet Protocol, it is not always "voice over the Internet".
First, most telephony system manufacturers now make their telephone systems using IP transport (internal LAN VoIP), although many small systems still do not. This type of VoIP does not save much money, but the phone system does not typically cost more, either. The true benefit to a VoIP base phone system is that you have easier options to integrate it with the email system and the desktop computers - in fact, many systems offer a PC based "soft-phone" to be used with a USB headset so that you don't need to purchase a phone (but the computer must always be running to get phone calls!). Some firms will get an open-source VoIP system - it saves money on the licensing costs (essentially free software) but it requires someone on staff to program and manage the system or you can contract with someone to provide those services.
The second way VoIP can make sense is if you wish to consider a "hosted" VoIP based telephone service. This is a service offering that uses IP to connect your LAN to their data center, where the telephone system core is located. This is usually proivided as a monthly service and can be less than buying your own phone system and telephone lines. Some of these companies then bundle it with Internet Access and other enhancements such as conferencing services. Of course, they also handle the LD, but as you mention that is not much of an issue for you.
Some people confuse the VoIP service from companies such as Vonage (which saves on LD) with true VoIP business service. Vonage and Skype are primarily for individual users and uses the open (and uncontrolled) Internet.
VoIP can be leveraged as part of a combined method to deliver telephone lines / service with local Internet access in a non-hosted configuration as well - this is the way cable TV companies are approach the telephone and Internet services they offer.
There is some truth to the "more than you need" statement - not knowing your specific requirements is a limit to any independent expert. However, based on the options outlined above, I believe you absolutely should consider VoIP based options, but it is not automatically best that you preclude other ways to work. Depending upon your location, I would give first consideration to a hosted provider - we have recommended that to a number of small to medium sized clients lately and it has worked well. At the same time, Open-Source based products (like the Asterisk) can save money if you have a local support firm that can be responsible for the care and feeding of the system.
