Output custom content not from the posts table?

Basically you want custom posts in WordPress that will be just pointers to external data?

If you don’t want full import then:

1. Store some kind of unique property identifier (that makes sense for your external data) as meta field or guid in custom post type.
2. Make wrapper function that will take ID of your custom post, retrieve identifier from it and use separate instance of wpdb class to query and return data from external database (assuming it’s also MySQL or you are on your own with retrieving that data). Use this in template as needed or append to post content with filters, etc.
3. Add some dressing, like periodic import of property identifiers with cron.