It depends on the date early or late war also on your preference. For my self if its early war 61 or most of 62 Frock. 62 thru the rest, sack for the field frock for more formal, full dress (Frock with epaulets) for balls and some parades. My sack coat is a private purchase 5 button sack coat with custom pocket arrangements. Again its your preference if you would rather use an enlisted fatigue uniform and attach your straps then do so.
As for the straps when I was "appointed" my surgeon gave me his old set of captains straps with the MS in them. I since use the style with the ms in them in order not to be confused with a staff officer when not wearing my green sash and in contact with the "brass" and not to be held as a combatant when in contact with the "enemy"
Assistant Surgeon USA