I understand that you are needing to do this with what you have, But its not going to work. You need more IPS at each location if your to provide failover. Is this for you and a friend or is this a small business that wants to run their own servers. You need at the least 2 routable ips at each loaction. I would recommend renting a server from one of the many of us out there on the net. Physical or virtual is your choice.
If your dead set on hosting your own at your locations then here is how you do it.
Offload your DNS to where you bought your domain.
Install pfsense and modify it to your suite. Here is a how to
Im not sure how your going to sync your data except for over the internet. Maybe you can use a VPN connection for the data sync.
Since your going small time you don't need much of a computer for the firewall.
Then just setup your rules to allow the ports (traffic) that you want.
But id just rent a VPS as this would be way easier.