I would strongly suggest you activate the Contact Form 7 plugin which has number of fields including the checkboxes to help you customize the contact form. Changing PHP files to enhance the contact form is not worth it since additional fields can get added. BTW, Appdev theme supports Contact Form 7, styles it reasonably well and hence you should have no issues creating one and checking it out.
About Mailchimp integration, I found this plugin that can be useful for you –
There are also other solutions (some involving code changes) – just google for “Mailchimp contact form 7”. Thanks.