The assembly includes: a front face; a structural front portion (4); a fender front portion (5); a flexible beam (43); and a headlight unit fastened to said beam (43). It further comprises: a centering and guide part (71) fastened to the structural front portion, in a position that can be adjusted, during assembly, along three axes (X, Y, Z); fastening and centering means (83, 85) for fastening the end (43B) of the beam to said part (71) and for centering it thereon; and guide means (93, 95) for guiding the fender front portion (5) on said part (71), providing a sliding connection slidable along one of the three axes (X), making relative adjustment movements possible in order to absorb deformation in said fender front portion. The invention is also relates to a motor vehicle including such an assembly.