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The rear pads are around $200 OEM, the fronts may be $198. I think doing brakes doesn't void the warranty..
As far as the rest, be careful with the bleeders, I would use a torque wrench to tighten them.
 

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You may have bad surprises with partial brake upgrades.
And as far as G Wagon, Mercedes likely sources the calipers.
A place like Brembo has 3 lines of product: OEM, GT & GTR.
If you look at the GT catalog and compare one caliper to another, often time they look the same from the outside. But the diameter of the piston is slightly different, or the radial position is different.
Even a caliper that is identical, if the point of application of the braking force is offset further from the center of the rotor, the resulting braking torque is quite different. Conversely, for a lighter vehicle, if the G Wagon calipers were to be associated with a smaller diameter rotor, the braking torque would quickly decrease.
In the Brembo catalog, the caliper application is designated through a range of rotor diameters.

As far as upgrading the rears only, there are a few factors there. One that should not be ignored is that the ABS system has its bias and calibration.
The way the ABS really works depends on how the car loses traction during braking.
During braking, weight transfer occurs. The weight transfer is directly dependent on the compression properties of the front and rear suspension.
If less weight transfer was to occur, for instance by double adjustable suspension, where you adjust the compression resistance to high, then the front axle having less weight starts to lose traction sooner. The ABS with multiple channels intervenes on the front brakes, while leaving the rears alone. But it relies on very high pressure and on a pressure reserve located in an accumulator.
Once the reserve in the accumulator is exhausted, or say reduced, the intervention on the rears, is much reduced, when it is needed.
The result? Longer braking distance. An upgrade with the shocks, turned into a braking downgrade.

What happens if you upgrade the front brakes only? without a corresponding suspension recalibration, the increased front baking torque, doesn't come with an increased weight transfer during braking and the overall weight of the car probably didn't increase. So therefore the front axle requires early ABS intervention, with the result described above.

What happens if you upgrade the rear brakes only? The rear brakes are designed to operate with a rear axle weight reduced during braking. If the rear brakes were to be more powerful (with more brake torque) than the front brakes and there was no attenuator, meaning the front and rear brake application would take place absolutely simultaneous, the car would swap ends under braking. I write this because I have seen this. I have seen this while working in a shop where a colleague had his dad's car which had been built in the 70's. bAck then there was a mechanical brake distribution device on the rear axle. You could use that to effectively get the rear to brake more than the front. The car swapped ends 3 times under emergency braking in the parking lot.
BTW, this is one of the reasons to have "slower brakes" on the rear axle: Drums on the rear, with disks on the front, floating calipers in the rear with fixed in the front. With disc front and rear, there is an attenuator for the rear.

So by upgrading rear brakes only, a number of bad things can happen:
1. The normal weight transfer under braking causes the ABS to intervene early, because the increased rear braking torque has the rear axle to lose traction early. By the time the front needs ABS pressure, the accumulator is now almost empty and the ABS is largely ineffective.
2. The rear upgrade is so dramatic (in the Japanese car forums, they tried to install front Porsche brakes on the rear car axle), that there is no more attenuation of the rear axle braking and the car swaps ends under braking.

I know this because I tried this, had adjustable suspension with partial brake upgrade on my Subaru. Had to let the car dive more under braking via suspension adjustment and even when I replaced the rear brakes to bring a balance to my brake upgrade, the programing of the ABS was for totally different brakes and it was only a partial success. I tend to believe that even if someone would grant you access into ABS programing, extensive research would be needed to reprogram it for your brake upgrade.
 
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