Pulling a vacuum and seeing that it holds is a good "gross leak" check. If you want to pressurize it, use dry nitrogen along with two ounces of R22 and then use an electronic leak detector. The leak detectors can find R22 much more easily than other refrigerants. This is actually a legal thing to do. The EPA figures that releasing two ounces of R22 is better than leaking a large quantity of 12.

This will allow you to check the entire system with about 200PSI of pressure.

Although on the 124 this is a time consuming procedure, if you have the MB service information printed out and close at hand, there is not really any mystery or magic to it. I have never pulled the dash from a 123 car, so I can't offer a comparison between the two jobs.

When I did my 124 I learned that it's pretty easy to pull the steering wheel and get it out of the way. Many of the guys here leave the steering wheel in place and just extend it as far as it will go before disconnecting the battery. My brother in law is an MB dealer tech and he encouraged me to pull the wheel. I was glad that he did, it made it much easier to deal with the instrument cluster connections and just generally be able to access things easier.

When removing the Allen bolt in the steering wheel center, make sure that you hold the wheel against your wrench, rather than just loosening the bolt with the opposing force being applied to the lock mechanism. This can in some cases put bur on the lock mechanism which will cause it to work poorly.

Good luck,