In Buxtehude's time the Magnificat was mostly performed "Alternatim", which means that organ versets substituted for the chant at certain points. However, it is very well possible that this Magnificat was meant for organ solo.
There already are some other fine recordings of this piece on contrebombarde.com, but since it sounds so convincing (IMHO) on the Zutphen set I thought I'd share it with you..!