Once you've identified a SMTP provider to use (which I can't help you with), an easy way to construct your email with separate plaintext and HTML parts would be by writing your own code for it.
Many programming languages offer support for this sort of thing; Python might be an easy choice because (a) it's probably already installed on OS X, and (b) someone already did all the work for you:
http://docs.python.org/2/library/email-examples.html
Check out the very last example on that page. It does literally exactly what you want. I myself run a version of that code every day. All you have to do is change the email addresses and the SMTP server near the bottom. (And the text of the email, of course!)
If you've never programmed in Python before, just copy that code verbatim into a text file, edit the file to your heart's content, and then type
python /path/to/your/textFile
in the Terminal to execute it.