Any shop should have the R-134A adapter fittings, and you will need them even if you go w/ Duracool or Envirosafe. You could also buy them at Autozone and screw on yourself, but there can be issues w/ the Shraeder valves. I recall a quirk on my M-B, like both low & high pressure R-12 ports were the same thread, though never ran it down.

If a Bay Area shop converts you to a Sanden w/ all new hoses, expect to pay at least $1500. Much cheaper if you do things yourself like I did, but isn't trivial. I don't notice better cooling in my 1985 300D w/ Sanden vs the 1984 w/ R-4, but never did a side-by-side comparison.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans