whoa! It is very significant. You need an experienced MB mechanic to run diagnostic on computer to pinpoint problem. It could be a cheap (sensors) or very expensive fix (computer). Sometimes it will cause the car to drive in limp home mode.
I have to be honest. Even thought I love the brand and believe the engines can run for 300,000 miles, unless you are getting this car real cheap, you have to think twice.
Even though everything is working fine now (assuming), many components are worn or due to be replace, it will be a continuing trend for this car unless the owner has previously replaced everything already or you are a very lucky guy.
For example, with that kind of mileage these are the wear items that you will need to worry about:
water pump
air con system
transmission
alternator
stereo amp
throttle actuators
all suspension components
engine and transmission mounts
etc...
Not trying to scare you, just painting proper picture for you. So if the price is a great bargain then it's worth considering. I personally would not hestiate in buying a high mileage car if the price is right.