taking over a new network site

Hi all,

My company has a central server network and we always keep the file servers in the datacenter and no servers are in the branch offices plus our users in the remote sites use remoteapps with their thin clients.
Our IT is taking over another small division and they have their own domain controllers and file servers on site. we are planning to have all the users access our remote apps eventually when their applications are moved to web and their will not be a need to access their local file servers. My question is below:

1. The small division has a good internet connection and they have given us an external IP address. We want to put a juniper firewall and establish a vpn connection without causing interruption to their existing network. My question is how can I split their isp internet connection and have a cable going to our netscreen.

I will put more questions but for now I just want to start with the tunnel connectivity at this small division.

Thanks,