To install the "city lights" or "parking lights" as are known in other countries should not be difficult since you have the wholes already. They use a simple 3W bulb.
In Euro/rest of the world version, the city or parking lights works much better than the U.S. "side markers + city lights" combo where the side markers are used, because when the main H4 bulb burns out, at least you have the dim light of the 3W bulb filling in the main bulb assembly.
That would be your only noticeable gain, because once you turn your dipped or main beam, the light of the main bulb overuns the little 3W light.
About your original question, the only way I found of gaining more power is adding 80/100W H4 bulbs instead of the regular 65/55W. But, be careful, as these wattage bulbs are not legal for road use in most states.
Some companies in Germany sell an aftermaket Xenon conversion kit (not just blue-tainted bulbs, but the real Xenon gas charged beams). The pair of headlights costs well over US$2,500 and you loose the high beam set up, so if you want to keep the high beam, you must order the AMG-styled bumper for the W124 which has openings to install "high beam driving" lights. Including the new bumper, the new high beam light, the Xenon set plus all wiring and labor should cost about US$6,000.
Simple solution: the 80/100W H4 bulb.
The wiring on the new W124 should withstand the higher wattage and temperature of this bulb, as long as you do not tend to drive long distances at night. On the other had, the same bulb fried the wiring harness on my previous W123 300TD.