How has the site been built? Sounds a daunting prospect having thousands of properties a week and being novices! Best of luck with it.
Sounds like a site of this size needs to be database driven, if it was (or is!) then you could use php or ASP to query the database and in effect generate a page per property, or link to an existing page via a stored url in the database. This would also make a sitemap easier to create as each record in the database (therefore each property) could be displayed on a single page via a repeat region with a link to that specific record in the database.
Well, that's how I would approach it anyway, perhaps others have alternative solutions?
Have you got a link to your site we could look at?