Custom changes to email newsletter plugin


I want to use the e-newsletter plugin to provide a email marketing type of service.

For this, I want to create a plugin (that works with the core e-newsletter plugin) that does the following--
(1) Provide 'plans' - i.e. one type of plan is defined by max number of end users (to whom emails will be sent) in a month/max number of emails in a month/both- user can pay for a plan via Marketpress (or other external script)-- and accordingly he is assigned a plan
(2) One question- can only an admin manage/create/send newsletters? or can end users also do this? How can an end user be assigned permissions to do this(i.e. what fields in WMPU database have to be changed/rows inserted to give a user permission to manage a newsletter)
(3) I also want the ability to use a service like Amazon Simple Email Service (or others like SendGrid) to actually send the emails.
(4) Can I create a plugin like the one above, keeping in mind that I do want the regular updates to the core e-newsletter plugin to work, hopefully without needing periodic changes to my plugin? What should I keep in mind to do this (i.e. to require no/minimal updates to my plugin whenever the core e-newsletter plugin is changed).
(5) Can you suggest any alterations/new steps to the above plugin,that will make it better/easier for me to develop it?
(6) Can you suggest/advice a different way to go about achieving the functionality that I desire?

Yours sincerely,