I can attest to 300E window regulators, but it is usually the rears that go bad...I have posted this in other threads, but to give you a heads up...
...the plastic track slider that is attached to the cable eventually self-destructs with age, and it will do so only after you engage the window switch. Eventually, you will notice that the window will not go up (or down) at all, which is an indication that the cable has now gotten snarled up. I have lost both of my rear units within a year of each other...
The plastic piece is replaceable (about $2) and removing the door panels is pretty simple (I've removed all four of mine and reinstalled in about an hour). A complete rear assembly goes for about $140, and as badly messed up as the cables were on mine, I just went ahead and got new units. Install is also a DIY, and can be done in a casual evening in about 1-2 hours...the only hassle is that the metal components and the door shells have many sharp edges, so wear hand protection if you have any.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car