I'm not sure you can achieve the following things all at once:

because what is regarded as an "attachment" depends at least in part on the
e-mail client used by the recipient - if they can only recieve plain text,
then pictures are inevitably going to disappear or be regarded as being part
of an attachment. And that applies to a lot of aspects of e-mail - you
simply cannot guarantee that what you send appears at the other end in the
format that you sent it.

What I can tell you is that there is enough in your list that Word MailMerge
"out of the box" is not going to help. If you can work with Word VBA, Doug
Robbins' macros at
http://word.mvps.org/FAQs/MailMerge/MergeWithAttachments.htm
are a useful starting point, but I think you are likely to run into trouble
trying to do attachments /and/  inline graphics. I have certainly had those
difficulties - it is possible that they are easily worked around using the
Outlook object model but I cannot tell you for sure.

As for using Outlook alone to do it, you can certainly do useful automation
in there, but I'm not sure you will be able to do everything you want for
the same reasons. But I'd ask in an Outlook group.

What I would probably do is
a. define a set of test documents and test mail clients that allowed you to
test all your requirements
b. acquire trial versions of 3rd-party solutions and email clients
c. see if any of the trials can cope with all the test documents, and if
not, think about what compromises you are willing to make.
d. choose a solution and live with it.