Larry's absolutely right, you need a professional vacuum pump for this job. I would also agree that since you are going through all this trouble, you might as well get your certification and buy R12. R12 will work better, no question. I got decent results with r-134a, but a big factor, if you read my link, is because I custom fit a modern, parallel flow condensor. Since you are using the original style condensor, you should stick with R12.
I had such a monumental task ahead of me when I started my project (100% system rebuild with custom parts) that I didn't want to bother with the certification, but now I wish I had just so I could say the job was done absolutely as perfectly as could be, and have optimal cooling in city traffic. If you drive mostly highway then r-134a is o.k., but in city driving it really doesn't cut it.
'84 300D, 173k
'90 300CE, 163k