I'm interested in purchasing this extension App Connect Form Validator, but I don't have any of the other extensions. So I want to be sure that I will not have to pay for any other extension in order to use this validator. I will be validating against database values (to check for emails and usernames) mostly. Is there anything else other than this extension and the free DMXzone App Connect?

Hello,
In order to check values in a database you also need database connector extension. It is already explained how to do this here: www.dmxzone.com/go/32764/dmxzone-app-connect-manual/32905/check-if-a-record-already-exists-in-your-database