Organize WordPress site, so it can maintain with huge database

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Custom Post Types are more convinient and easy to implement, so I suggest you use them.
Also the Posts will be ready to make a blog section in the future.

About optimizing.

  1. Use some caching plugin, there are lots of them in WordPress Plugin Directory.

  2. Turn off the revisions to save database size: add define ('WP_POST_REVISIONS', 0); into wp-config.php

  3. And try to use as less plugins as possible. Only what you really need.