Mailchimp (or Mandrill if you want the API backend) is free for first 12k emails per month. If you are only sending it once, it shouldnt be an issue.

Better to use that then SendGrid since it'll less likely to hit spam folders.

It does sound a bit like spam since this will go to every single email account that has mailed you or you have mailed before. Would it be better to just use Facebook / Twitter / Linkedin + email ~200-300 people.