That wasn't so bad
I think you can graft the lower plastic of 88-up door panels onto your door panels so you keep the matching perforated leather and get the door speaker housing. I don't know if the airbag car glove box is part of the console or a separate piece.
If you're in Berkely, go by All Mercedes on 14th St. in Oakland and poke around their yard. I'm sure you'll get the info you need to decide how far you want to go with the update.
FWIW, I find the 88-up seats more comfortable than the 81-87 seats. It's not the pleating, it's the shape of the seat.
Not all 91 126s had passenger side airbags. My 91 300SE doesn't have a passenger side airbag. I'd like to fit a passenger side knee bolster. I think there'll be decent access to the glove box with the bolster in place.