The evaporator failure is not something you can predict by age. Some of them spring a leak and some don't. My '88 300E still has the factory evaporator and it's still holding tight (knock on wood.)
As far as reverse converting to R12, I would highly recommend it if you are opening up your system to repair a leak or some such. I just went through the conversion procedure in another thread at the top, I think the title was "R134 Conversion" or some such.
You can indeed buy R134 UV dye with a little R134 in a can. For R12, however, you will have to get pure dye. They do not sell R12 in a can with UV dye, because they can't legally sell you R12 unless you have a 609 certificate.
Go to your local auto a/c supply house and they will have a kit you can buy that can put dye into any type system.