Hi There, I am an Aussie living in the Philippines and this is a cool project. I have a great deal of mapping and forum experience.
I would develop this as follows:
The map would be embedded on the homepage of the Forum.
When the user types in an address, it will be Geocoded to arrive at a point and then drop a marker on the map. Any earlier pages already on the forum that are close by will also display as markers on the map. A database lookup is performed for the address to see if there is a forum page already for this address, if not, it will create a new forum page. The use will then be redirected to the page. (after logging in or joining)
SEO would be handled by the forum using a MOD_rewrite package. This would make the address of the building the url to the discussion on the Forum and available on search engines.
My preferred Technologies for this project would be (All Open Source with No Fees)
OpenStreetMap
Nomination geocoding Engine (Could use Google but Api limits apply, possibly Google fallback if Nomination lookup fails)
PostGreSQL (Database)
OpenLayers (Map Display)
PHPBB Forum
You would need a server to run this, either a Windows or a Linux Server will do. Root Access is needed. I would be willing to develop this on my servers to start with for testing and move to your server when done.
I would have Skype available during the project for discussion
Best Regards
Mark Cupitt