I have an 09 350 slk. with around 79K.
My dealer has recommended have the brakes flushed. They want $400.
I contacted another dealer (much smaller company) and they told me they don't recommend the service unless I was having issues (which i am not). They said the brake fluid is guaranteed for life.
I bought the car with 49K and I don't know if had been done before.
This is one of those things where European and US manufacturers often disagree. While the brake fluid might not "wear out," it can become contaminated with water and wear particles from the various moving parts in the system. Likely your car is using DOT 4 Brake Fluid. The following information from Wikipedia is a good staring point for learning more about brake fluid (https://en.wikipedia.org/wiki/DOT_4):
"DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 3, 4, and 5.1 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is usually not much of an issue as the brakes are generally not used so hard, but can be of serious concerns in police vehicles, race cars or motorcycles due to the higher heat generated during their much more aggressive braking, and trucks where higher heat can be generated while braking hard and fully loaded."
In addition to the concerns regarding the lowering of the boiling point, water in the system can also promote corrosion, although I have never seen this as a big problem on modern cars (it used to be a real problem with older cars with less well sealed braking systems).
I assume that Europeans manufacturers (particularly the Germans) recommend the changing of brake fluid because they are more concerned about brake fade than manufacturers in the US where typical speeds used to be significantly lower, and therefore less of a demand on the brakes. These days speeds in the US are much higher than just a few years ago. On the other hand, most modern braking systems are much better and fade is not as big an issue. The Japanese recommend brake fluid changes because it is a Japanese thing to do.
Personally I change my brake fluid every three years. It can't hurt and I feel that it can help keep the components in the brake system in better shape (I worry about the effects of moisture on the very expensive ABS pump). On the other hand, I do it myself and brake fluid is cheap. A $400 charge for changing the brake fluid seems ridiculous. 1 Liter of Pentosin 1204116 Super Dot 4 Brake Fluid costs around $13. The garage will likely use a pressure bleeder (the Mercedes recommended method). This connects to the master cylinder reservoir and pressurizes the system. The car is lifted in the air and the mechanic walks around to each wheel and opens the wheel bleed to allow the old fluid to be pushed out and replaced by the new. Start to finish it can't take more than an hour. Even with $120 labor rate, taxes, disposal fees and equipment charges, the whole process should not cost more than $175 at a high end dealer.
I changed my SLK280's fluid earlier this year using a vacuum system and a floor jack and did it in less than 2 hours. The vacuum system is much slower because you have to attach it to each individual wheel and "suck" the old fluid out. Since you are only working with atmospheric pressure to force the fluid through the system, it is much slower. I also had to use the floor jack to lift the car at each corner and remove and replace the wheels one at a time.
You can also just use the old manual pump the pedal system to change the fluid, but that takes two people and a lot of leg work.
I think changing the brake fluid is a good idea and that you should be able to find a decent independent shop to do it for a lot less than $400.