I want to wirte addon for Thunderbird

Hi all!
First short about me i am freelance java developer with background of writing Java enterprise apps.
Due to my activity sending email to my clients i thought about to automate this process.
So i want to write following addon for Thunderbird

  1. Read from given directory the sub directory’s txt file 3 lines:
    project description
    Salutation (For dear Sirs or Mrs)
    Path to file with my profile.pdf
  2. read from another txt file the email and the text
    the text consist place holders for the line which i have read previously.
  3. Create draft with inserted lines (replace place holder with given text), so the draft should include all previously given information (PDF should be included as attachment too)
    This addon should allow to send only 10 emails per execution of the app , basically it should have a list which show how many and to which emails the email was already send (checkbox like for checked for already send and not checked checkbox unchecked)

What kind of sdk i need to use and what kind of thunderbird api i must need to read to be able to write this app.

I suggest you look at this existing add-on https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/

I think i solve my problem, basically i wrote a Java programm which reads the eml file (generated from Thunderbird) and replace the text with my text which my java programm read previously, one manual step would be just to attach the pdf file when the eml file was open by thunderbird in edit mode.
Yep, i have written a Java programm which can edit eml file, and put at noted place customized text from another plain txt file.

You should share your program on GitHub, maybe it would be useful to someone else!