Call me simple, but could the smell be simply the buildup of microorganisms. I saw on a car show that the best way to rid the smell is to drill a hole in the duct and then inject this anti-microbial creme (looks like shaving creme). After a while the creme will kills all of the bacterial and wala no more smell.
The quick and dirty way is to spray something like Lysol into the duct (lift up the hood and turn on the a/c) and spray underneath the windshield wiper blade.
I think, not sure, of the disadvantage of converting to R134 is that although it is much more environmentally friendly, it doesn't cool the air was well. If you live in a cool state, no big deal, but in NC, you want the AC as cold as possible, especially this week!
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)