If you have a mail from party A, part of which you want to send to party B, the obvious solution is save the attachemnt, then send a new mail with the attachment added.
It is always best to send a 'clean' mail as said above, but if you're in a hurry you can forward it and highlight all the text you don't want to send on, then hit delete.
Mind the rubbish on the bottoms of forwarded mails (incredamail and antivirus messages etc) is sometimes hard to delete.