Get something else. The ML-Class is Mercedes least reliable class. It was plagued with issues since the first model from the late 90s. It looks like you possibly found a first model ML-Class, which will give you A LOT of problems.
Keep in mind, Mercedes are expensive to maintain and repair. The average oil change for a Mercedes can easily run $200.
the mileage is ok for the age of the car but just remember that any major repair will cost more than you paid for the car. new tires will cost half the price of the car. insist on a car fax and have a mechanic check it out