You're getting lots of good advice here. It sounds like Moab operates as I do. I will buy tools and equipment to do a job when I could pay to have it done for less money. After I'm done I then have the equipment for the next time.
There will be MUCH oil in the evaporator, it really should be flushed. Since the lines are right next to each other after removing the expansion valve, the rag suggestion will work pretty good. I've done this several times before on 123 cars and have never made an objectionable mess.
If you choose to not follow our advice here and use the venturi pump anyway, make SURE you have enough compressor to keep up with it. It takes LOTS of compressor to keep up with it for the length of time that you really need to evacuate. As mentioned before, do this in the warmest part of the day with components as warm as possible. Don't try it in 60 degree or colder weather.
Finally the a/c systems in the 123 cars are some of the worst candidates for 134 conversion. Your SL is a different animal all together and has a much better chance of successful 134 conversion. The capacity of the 123 a/c is adequate with 12, but with the 20% loss of capacity when going to 134 it will be marginal. The only car I know of that is a worse candidate for conversion than yours is the 124 MB, it is virtually hopeless for conversion.
I can tell you are dead set on 134 and that's okay. We are only trying to share our experience(s) with you and help in any way we can.
Best of luck with the project.