Yes, you can use an existing domain with your G Suite order.

Each email account comes with 30 GB space.

Your email can be accessed using any Smartphone or Tablet. The Gmail interface is compatible on all major Operating systems such as iOS, Android, Windows Mobile, Symbian and Blackberry.

Yes, you can. Use Gmail's vacation responder to let people know that you won't be able to get back to them right away. While your vacation responder is turned on, Gmail will send your reply to people who email you.

When you sign up for a G Suite account, you agree not to use the account to send spam, distribute viruses, or otherwise abuse the service. All users on your domain are subject to these agreements, which are part of the G Suite Acceptable Use Policy. If Google identifies a G Suite user who is violating these agreements, we reserve the right to immediately suspend the user. If the problem is domain-wide, we reserve the right to suspend the entire account and deny administrator access to all the G Suite services. In such cases, we send a notification to the registered secondary email address for the domain administrator.