There is a pretty good list of common issues for the W210 at BW. It's in their stickies.

The vacuum thing is pretty easy to test. Check the line from the PSE under the rear seat to the trunk, you can just suck on it. If it releases, the connection in the pump itself may be flaky (disassemble and check/replace the hose). If not, it is more likely to crack at the connection than along the way, but if it isn't cracked at the connection I'd replace the whole line.

If the AT hasn't been serviced (and you buy it) that would be my first priority.

Good luck.