To begin with, Mercedes engines do not have timing belts. They have timing chains.

This engine is an interference engine which means that if the timing chains break, it will cause serious damage.

If the timing chains break, the cost to repair the damage will probably be more than the car is worth.

How many miles on the engine? How many miles since the timing chains were replaced. Mercedes used to recommend replacing timing chains if you have more than 100,000 miles on them. I'd say that if you have 150,000 miles since the chains were replaced, you should have it done.

I recommend you take the car to a dealer who has a good reputation and ask that they check it out.

$600 to $800 seems reasonable to me because it used to cost well over $2,000 to change the timing chains on older V8's from a simplex chain to a duplex chain.