are mercedes expensive?

The 6 cylinder engine types are not that costly, but the 8s can be expensive, depending on the model .  The MB is a well built car, so you can expect to pay more than for  other cars,

On the question regarding keys and the car stopping (stalling) that could be the locking system.  There is a theft protection on most MB that engages if unusual activity is detected. The locking system has to be reset. It is to prevent a car thief getting away easily.

Yeah,   both buying one and parts are expensive.   If you know a good local mechanic who works on them you can save some money VS taking it into the dealer.  

They are in the price range if a buiks around 35 grand 

Yes.  The purchase price is not the only factor.  Mercedes owners are expected to have the money to pay a lot more for repairs.  For instance the battery in my Mercedes is $500.  An oil change is about $240. 

Compared to most other major automotive brands, yes.

yes and the repairs

Yes. They are a luxury brand. But you can pick up good used examples that have been properly  maintained for reasonable prices.