swiftmailer - html and email templates
Swiftmailer just sends emails ... it doesn't let you create HTML / plaintext content for emails.
For ease of setting this all up you likely want to use a module that supports the Mime Mail module and then at least use something like Previewable Email Templates (PET) module. Quote from the PET module page:

The Previewable Email Template (PET) module lets you create email templates, with token substitution, which can be previewed by the user before sending. The emails can be sent to one or many email addresses in a flexible way, and the recipients may or may not be Drupal account holders (users).
PET stores templates in a db table, not the variables table, so there is none of the memory usage which goes with the latter.
PET templates are in one place for easy management by site administrators. Emails can be sent one of three ways:

interactively, from a link.
programmatically, in response to any situation.
via a Rules action (D7 only).