I'll save you some time on ripping apart the door panels, not much to be saved there.

The bulk of the weight is in the padding under the carpet. I removed half the drivers side for a Sparco install and it was about 20 lbs.

If you are looking for a small coolant tank look at the CLK320 ones. They sit in the same area, but are much smaller. You can also look to go completely custom and get something like we did for the E55 build.

An Optima Red Top will save about 22 lbs and fits perfectly in the area near the vaccuum box in the trunk. Mock up a bracket it to secure it and your good.

There really isn't all that much weight to lose for free though. After you strip the basic stuff swap to some light seats and free up another 75 lbs or so. Then move onto some wheels.