From my days working in an office environment, I would say no. A memo usually has the To: and From: lines at the top, along with a Subject: line (much like an email). When I wrote/received memos, the only thing that went in the body of the actual memo was the information you were delivering/reading.
"Dear Sir" and Yours sincerely" are normally used for formal letters, not memos.
The "legal" status? Are you writing a legal memo? If so, then I have no experience of that. I didn't work in law. Perhaps someone else can enlighten both of us on that point!
I was basing my answer on my old Civil Service job where, to be honest, once computers were introduced to the work environment, memos became somewhat obsolete as they had always been handwritten/typed.