Hardwood flooring is not recommended in a mercedes especially the older 300D. A known issue is the water buildup in the rear seating area when the weep holes get gummed up. A nice brazilian walnut with an epoxy finish may work but also install an underlayment preferably felt paper. A lot of folks use the red rosin paper to absorb moisture and stop squeaks but felt (tar paper) works also.
It would be nice to have your floor match the dash trim and the of course compliment the curtains. The pre-finish flooring has come a long way and now the finish is impregnated into the wood. So that might be an option as it will be very difficult to get an F-89 into the back of a 300D to sand the floor.

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