The 15-91 is considered by many to be the most "heavy-duty" of Singer's domestic machines. It and the 201 were sort favourites of tailors and dressmakers. That said, these are not industrial machines by any stretch of the imagination though, nor were they intended to sew heavy canvas or nylon webbing on a regular basis.
For heavy sewing with heavy threads you may well need an industrial.
If this is a commercial enterprise, you may well find that any domestic machine you try, including a vintage such as a 15-91 will need so much fiddling with that it's just not worth the effort.
Last edited by 1.41; 06-04-2022 at 10:21 PM.