You've done good so far
Brother of The Benz, Johnson
You did good replacing the drier reciever, it always helps when servicing an older unit.
But STOP at this point and return the balance of the parts.
Beg, borrow or rent a vac pump and gauge/manifold set to evacuate the system.
When making the change over keep the R-12 high pressure service port as original for in that way you can use older R-12 gauge/manifold for pumping the system down from the high pressure port.
Your change over kit should have the service /charging hose for the low pressure side.
All that you should purchase now is the charge capacity gauge/hose and fitting.
If you can't find the "Hot Shot", contact me on my E-Mail and I will get it for you here in Houston and mail same.
I pray someday your problems all are corrected, for you have had your share.
You are the Classic Mercedes Benz Lover.
Happy Trails Beep Beep from The Spiderman in Houston!!!