I have used the Wurth products and are good. The SKS stone guard is water based but is extremely durable. Must be sprayed with a gun and depending on which gun you use, can be sprayed more fine or course. The stone guard spray can is a fine texture and solvent based and also good. I bought beige sometime ago and yes, is now discontinued. I believe the original undercoat was beige in color as I saw on my car. Because of that, I would be tempted to use the 3M and should be just fine. Most of their products are good.
From what I saw on my W111, there are places where the seam sealer is brushed on and places where the undercoat is sprayed on. Kind of a mixed bag.