You can find new and used Bundt-style wheels if you look hard enough. THey are far more common in Europe, so it helps to have connections over there.
Expect to pay $1,000-1,500 for a set here, used. They can usually be refinished to like new condition. Look on the
www.m-100.cc board and make the inquiry there, and someone with a set will offer them to you I'll bet.
There are factory 15" and 16" steel wheels available that will fit the 6.3. The 15" wheels also had a factory steel hubcap available that matches the hubcaps available at the time the cars were new. I don't believe these hubcaps are currently in production, but again they are out there if you know where to go. The wheels are available from any Benz dealer and run about $150 each. The hubcaps ought to be $30-50 apiece. This is for a 15x7J steel wheel. The part number is something like 126 400 XX XX but I don't remember what the last four numbers are.
There is also a 16" steel wheel available, #129 400 02 02, which runs a little over $200 each. There are no old-style factory hubcaps available for these wheels, but I believe that MB did make various plastic hubcaps available in various styles that will fit these wheels. I have a pair of these wheels that I use for my 1994 E500 upon which I have drag-racing tires mounted for track usage.
All in all your best and cheapest bet is to go with the 15" steel wheels, if you can find a set of hubcaps. Again you ought to be able to get them from someone on the
www.m-100.cc bulletin board, as that is where the M-100 crew hangs out these days.
Good luck!
Cheers,
Gerry