"I started by looking for pre-WW2 Sauer & Sohn models, however they are really hard to find and command extremely high prices when available in mint condition or have often been altered. I subsequently noticed that immediate post-WW2 Sauer & Sohn production at Eckenförde (then in Western Germany) had almost the same characteristics as the pre-WW2 one, was slightly more available as well as more affordable and I therefore re-directed my search."

unfortunately its true that pre ww2 S&S Kipplaufbüchsen are "overprized" today, they were rare than and now and build on special order for wealthy gentleman so made in a high standard. not alone this there are some idiots around today who are able to screw a weaver rail and a plastic stock on this.

much underrated today are Sauer&Sohn Tellbüchsen build mostly for today obsolete caliber and not exactly a high power "Karpathenbüchse".
something like this http://egun.de/market/item.php?id=6978547

this Tellbüchsen were affordable and light build single shots