You seem to have an inaccurate impression of how networks work. Filtering doesn't require knowing what your IP address is and DNS is not an effective means of filtering content. Filters work by checking the content being requested, not where the content is being requested from. Using a DNS server that doesn't return results will stop a very basic attempt at access to a site, however it is easy to get DNS information from a different DNS server which will allow the DNS filter the be bypassed easily.
In your case, it appears that OpenDNS offers a DNS server that won't return results that match your filters when requested from your IP. You should be able to run the updater on more than one system and can probably get a router that would support making the updates for you automatically.
That said, I'd re-emphasis that this is not an effective means of filtering. It is a trivial thing to make requests to another DNS server and then requests can be made to a "blocked" server without issue. It is no deterrent at all to someone that knows what they are doing and you should expect that someone who wants to bypass it will figure out how to.
The two DNS addresses is just a redundancy standard. Generally at least two DNS servers are entered to make sure that you can still resolve websites if one of the servers is down.
A static IP is easier to work with, but this is not a reason to buy one if you don't have other needs for a static IP. (The main, and really only, reason to need a static IP is if you want to be able to host information on the Internet. And even that situation can be worked around with services like DynDNS.