I'm willing to bet he put in R-12.
In fact, I'd stick with R-12 as long as possible. If you convert to R134a, it'll never blow as cold. Conversions never do.
I hope this works out for you.
The W126's are widely regarded as being one of the best Mercedes' ever.
The SE's have a shorter wheelbase than the SEL's, so it may help you fit it in your garage.
Let us know what happens.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".