W124 300E 2.8 RPM fluctuates with AC on in D gear

[shoppingshzhang]

I recently bought a 300E 2.8 with 138k miles.
Everything works fine except two things.

1. RPM will go from 600ish up and down when AC is on.
I don't why this is happening. Any has any ideas? The car has the automatic AC option.

2. Rear brake makes grumbling noise when pressed.
I will probably take this to a shop.

Thanks!

[shoppingshzhang]

Some photos. Need to fix some of the rough edges
http://imgur.com/a/5yILV

[mespe]

if the idle fluctates when the A/C is on, it could be that the car is low on freon, or has moisture , causing the conmpressor to kick on and off.

noise from the rear brakes probably means it's time for new brakes.