A common and well known problem with R107 models is water ingress into the front footwells coming through the bulkhead/ heater plenum chamber.
This bulkhead area can usually be repaired by fabricating in a new section and cutting out all corroded metal.
With improved material and methods of sealing, priming, painting and treating metal than were available when new this now becomes a once only permanent repair.
Water ingress to the footwells can also occur through internal corrosion of the drains in the side of the sill panels. Once the drain tubes have rusted through they allow water to collect in the sill panel lower seam area to the floor pans, eventually rusting through to outer edges of the floor pans to sills.