From my own experience (with Entourage, but I think that Mail will be similar)
To view the whole of an email I think that the solution is to
select it
use the menu 'Data Open with 'and select the mail application
One can then open the attachment from within the mail application by double clicking it (it launches the application that can open the attachment).
I don’t think you can search text inside an attachment.
There is a paying app called ThinkArch that can store the mails and attachments as separate pieces, not sure you can search inside these attachments either.