The information from the dossier came from the Russians, so if you are investigating Russian influence in our elections wouldn’t you go down that rabbit hole as well to see where Steele got his info from and whether or not he was truly investigating or was a mule, or at worse a co conspirator.
YES, to do a minimally competent investigation you would have to try to get at the origins of all of the allegations and info in the “dossier” as well as the physical creation and various transmissions of the dossier document itself.
Otherwise, you have not idea whether your whole investigation has been “played” and manipulated by some person(s) in the background.