Colin,
They probably buy the pre cut and sewn seat covers/carpet from the same German or US suppliers as any other upholsterer. They need to understand the differences between 230 and 280SL's The most difficult parts are the dash and soft top hatch and so long as they know what they are doing and have done it before they should be fine. I have used two different places, one was Sale Motor Trimmers, Sale who did a 280SL for me and the other was Reinhard Klavers but he did not recover the dash, only recoloured it.
Garry