With enough money anything is possible. If you've studied the SEC page you'll see that there is major wiring that must be dealt with. I think this is harder to deal with than the actual fitting of another type of engine.
I think the 97' E420 engine and tranny would really be nice being that you'd get better MPG and have a nice smooth shifting 5 sp tranny. Personally I find the performance of the previous generation MBZ unacceptable!