The OEM wires are solid core(actual wire)with resistor ends to prevent radio interference. On most Mercedes cars that means 1K Ohms of total resistance. The system only puts out enough current to overcome the single largest resistance in the circuit. Usually it's either the rotor gap or the spark plug gap. Some of the aftermarket wires have artificially high resistance causing the ignition system to put out more current to push the spark through the wire. This is the way most American car systems were designed.
Unless the wires are damaged you can replace just the spark plug ends. If you need wires, BERU seem to be the best. Even though Bosch does produce some of the OEM ends, the "consumer level" products they have stuck their name on in the past were made very cheaply. Though I'm not familiar with Magnecore or Nology wires, I do believe they operate on the "American car" theory. It's best to use OEM type wires and open the plug gaps a bit(though you may lose a few Mph off of top speed). It makes them idle much smoother.