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.

ONLY very expensive when BRAND NEW! As they AGE they get CHEAPER and CHEAPER Whoever paid $500 for a BATTERY needs a good psychiatrist! It's RIDICULOUS and OIL CHANGES are the SAME using proper BENZ approved oils, like MOBIL ON0W-40 and a FLEECE filter for about $60! That poor guy is GETTING ROBBED! AMAZING isn't it? I bought TWO BENZ cars used for around $3,000 a piece and they BOTH were TOTALLY AWESOME! Better than most NEW cars and lasting indefinitely! Brake pads for my C300 and C280 were only $25 per set rear and $45 for front. BEST OIL CHANGE is PENNZOIL ULTRA EURO 0W-40! But BENZ uses SEVEN QUARTS< not FIVE. I LOVE the fact that these cars depreciate! RIGHT NOW looking at CARGURUS and AUTOLIST< there are many for under $10,000! some ARE ONLY $8,00 AND THEY ARE still ABLE TO BE financed AS WELL! there IS A  million mile club FOR THEM  They are the ONLY CAR I know of that if you use the AC system you get BETTER GAS MILEAGE! This is because they run the FUEL LINES through the ICY COLD AC hose in a special chamber. This CHILLS the fuel and it gets MORE ATOMIZED! ALSO I use DENO TWIN TIIP IRIDIUM Plugs in ANY car I have, a s they improve POWER and MILEAGE by as much as 20%>>>the oracle IS totally insane! benz PARTS ARE cheaper THAN gm PARTS AND MANY gm CARS ARE european DESIGNED AS well! nobody PAYS $500 FOR A DAMN battery GET A GOOD shrink iF YOU ACTUALLY pay that much! The BENZ hoods ALL go STRAIGHT VETIVEL to[protect TALL people like ME from hitting the HEAD on the HOOD!  I LOVE BENZ CARS in general but AUDI will ALWAYS be my FAVORITE! THEN BMW than FORD.  I MISS COLUMBO! SORRY he DIES! SAME for SEAN CONNERY! JEEZ!

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.