I have just read your question and by now you either repaired or sold your ML500. However here it goes in case is still helpful. There is almost no difference in replacing the packings in the rack and pinion and the rack and pinion itself - I am assuming your ML is leaking Pwr steering fluid at the rack and pinion- the only difference will be the cost but the job of replacing the packings (o-rings as you name them) is even tougher: In both cases you have to remove the rack and pinion assembly -which is tough. If you replace the whole part you just take one off and put the other on; if you want to replace the packings (around $80 Dollars a kit) you have to disassemble the rack and pinion assembly and replace all packings -some are o-rings and some are packings- and you will have a 50-50 chance it is going to solve your problem. Let me know what you decide! Good luck!