Every situation is not the same.  Rotor can't go wrong unless if you have
stuck(lockup) brakes in the past.  You should hear loud noise when you
brake(like your finger nail cratching on the chalk-board), if your rotor or
brakes are in bad shape.

Yes you could if you have some basic experience on how to replace your
brake.  Rotor usually slide offs, but do not replace it now,  Do not listen
to MB service people, they are hungry, they lied to you.   I can tell by the
mileage of your car.

In my opinion, rotor should last at least 100K miles, a little longer than
the brake.  My C280 has 80K miles right now, and my front brakes are good
for another 30-40K miles, to give you an idea, 80K+30K = 110K, that's how
long my C280 brakes should last.  It depends on how much brake you used.  I
say you should not have to worry about your brakes unless it makes loud
noises, or it doesn't brake well for you.

No, not to me, brakes cost so cheap, front brakes are easy to install.
Assumming brake costs $150 a pair, labor should be 3-4 hours at the most.

I have no experience on the latest C class but I do own and maintain its
predecessors: 84 190D and 96 C280.  I think 20k miles is a too-short
distance and $1.2k is a too-expensive brake job.

The disc brake is easy to replace, much easier than drum brake.  I can
do both wheels in 2 hours so dealer must be able to do that too.
Suppose dealer's hourly rate is $150 (very high end).  Does that mean
they charge the parts for $900?  That is enough to buy parts for
complete rebuild for all 4 wheels.

Note that I assume your situation is the normal case (due to the low
mileage).  I can't provide precise recommendation without seeing the
car.  Since the current estimation is too much way off, second opinion
can help us (the group) help you better.

Disc brake is pretty easy but I would not suggest you to start from it,
unless you have done other jobs like oil change, spark plugs.  You do
need decent tools: floor jack, stand, a set of metric sockets, 8"
C-clamp, etc.  The bolts are probably 15 or 17mm, pretty tight.  If you
think you are capable of doing that, contact me via email.

Many online places offer good deals on MB parts.  If you are at west
coast of US, autohausaz.com usually has the best price.  If you are at
east coast, buymbparts.com is pretty good: competitive price with fast
shipping.  They both carry OE or high quality parts.  There are other
sites but these two are the ones I use most.

I'd say that spending 1,200 dollars on brakes when you can't detect
any problem is not a good idea. The symptoms if the rotor is warped
are pulsing in the brake pedal or steering wheel while braking.
Alternatively If the rotor is worn down below spec, it might give no
indication, yet should be replaced. However this is only something
that would happen if you were to drive with brake pads that have worn
past the point that there is any friction surface left, and you were
using the metal backing as brake pads. This should not be the case on
a car with that many miles unless it had a manufacturing defect. The
only other case I can think of would be a rotor that was cracked, in
which case they should be able to show you where it is bad. However,
since MB rotors are so meaty, I really doubt that they would crack. In
short, the odds of this mechanic being legitimate is quite slim. If he
is, that estimate seems quite high. I'd look for another place to take
your car.

Whenever I buy a make of car that is new to me, or move to a new
location, I ask around about good auto shops in the area. Try asking
people in parking lots with similar cars. In your case perhaps asking
the guy with the 5-15 year old mercedes in the parking lot if he has a
favorite mechanic that he trusts is the way to go. People with cars as
new as yours might not have required enough maintenance to get a
feeling of which shop is credible. I have found three wonderful shops
using that tactic. Neighbors are fair game too.

Changing rotors is not a hard job if you have spun wrenches before.
I'd be urprised if you couldn't find a good tutorial on it online.

Good luck,
Bill

Everyone here offered excellent advice... and the story is turn down that
offer from the dealer until you personally inspect the brakes yourself.
There is no need to change the rotor at such a low mileage unless there is a
problem. Pads are the wear and tear item however... you must inspect them
first.