Suspension Problem
 

I drive a 04 E240 (non-airmatic) 120000 KM and feel some suspension problems;

1. When i get past a big hump, the front part of chassis hit the ground pretty hard. this happens when i have 3 or more passengers on board. Have my springs gone weak??

2. While coming out of a turn, my steering doesn't completely set back to the center by itself. i manually rotate almost 10 degrees to bring it back to the center.

3. i installed brand new tires, but have a vibration over 60KPH. Balanced those even with road force balancer but in vain. One technician suggested to change the Idler Arm. does it make sense??

please help

No, it doesn't make any sense. Your car has rack and pinion steering and as such has no idler arm.