I have a 87 300E that burned out its coil. I replaced the coil (twice, thinking I got a bad one from the store). I've got 12v at the primary, but still can't get a spark from the plugs. I'm at wits ends about this. A friend suggested a security lockout. I've worked on my own cars since the 70s, but not on MB before. I love the car, but not when it's sitting in the driveway driving me crazy like this.

 

To get a spark you require an INTERMITTENT 12v supply to the coil.
If you have 12v across the copil this is a good start but the question is whether the 12v gets interrupted as the engine is turned over.
The -ve (usual) connection is made via circuit which is interrupted in line with the crank rotation / number of cylinders.
This circuit used to be achieved via old fashioned mechanical contact breaker (points). These became electronic in the interests of reliability and accuracy and nowadays the electronic circuit has become much more sophisticated with the introduction of immobiliser technology (to prevent theft).
Many MB's nowadays also have two plugs / cylinder designed to "fire" at slightly different times in the cycle.
Your friend may well be right in thinking that the immobiliser is operational.
I'm not familiar with the exact spec of your car.
hope this gives you a bit of background.
Stuart