If you want to move to VoIP, you will need to put some type of WAN/IP network in at each location. You need to check the cost of doing this. There is going to be the intial startup cost for equipment, line installation, etc., then the monthly recurring costs for each IP line as well as the cost of maintaining the network which will require someone to do this.
Then you need to start pricing the VoIP equipment that will be needed. Sounds like you will need just gateways to, no need for gatekeepers, and it sounds like they will all need to be analog.
If you want to discuss this more, you can contact me at email@example.com.