Sounds like a trip to bullsh#t park to me.
Is your car subject to harsh salt environments? I might agree with worn suspension components, but I thought the technology used to rust proof undercarriage components pretty much proved successful in the 80's!
My 73 Bug spent most of it's life in Illinois, and at least the first 15 years of that were daily driven, including salt laden winter driving! When I turned it show-worthy, the only bad rust areas were the body surfaces, and the rocker panels. The frame had some slight surface rust.
I would have them SHOW you the rust on the frame, then take the car somewhere else!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car