The 722.6 electronic transmissions are designed and factory assembled with rather loose tolerances. As the transmission wears, there is little-to-no reserve to fall back on and problems (such as excessive output shaft play) can result.
This design weakness became apparent shortly after the 722.6's introduction and a LOT of these transmissions have been rebuilt under warranty.
Also, this transmission utilizes an "lifetime" transmission oil that requires renewal every 100K. I am aware of several 722.6 owners who, against dealer advice, have renewed their trans oil as early as 40K miles, in an effort to minimize wear and extend transmission life. At approx. $14/qt, fluid renewal costs over $100, but so far none have had any AT problems. Granted, a small sample size, but something to consider should you acquire the '97 E420 (or any M-B with a 722.6 transmission).