Ron

I have a 1997 E420 and a 2002 E55. The W210 is an excellent car. Silky smooth, nice handling and very reliable. Overall I've had very little go wrong with my E420. It's like a Japanese car, you just change the oil put air in the tires and drive.

I'm not too familiar with the v6 engines but I can tell you about the chassis and transmission.

1) - Transmission, i was designed as a sealed for life unit but could use a fluid flush and filter change. The 722.6 needs special fluid , use the MB approved stuff.
2) - Transmission conductor plate - Can case limp home mode if it goes bad, replacement is about $150.
3) - Brake switch - can cause limp home mode and ASR light to trip. Replacement is cheap
4) - Rust, check everywhere, specially spring perch and behind the rubber door seals.
5) - Sway bar bushings - Make an annoying rattle
6) - Control arm bushings - Cause a shake at 70mph +/- that feels like out of balance wheels
7) - Blower motor regulator, goes bad. I think the 99's have the updated unit

Aside from this, it's a very comfortable, well built and reliable car. As it's an 99, it uses OBDII which makes diagnosing problems easy.

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With best regards

Al