When you delete the temp domain you will need to delete the website, any mailboxes, and the domain itself.
The Database and FTP would be unaffected by this and the files themselves for the website would remain in your Hosting Space on our system.
I would suggest though not using a temp domain to setup and test the website and instead setup the website with the domain you are actually going to use. An Instant Alias domain is created when you add your domain so you can test the website is setup correctly before you then point the domain at our network.