Philips screwdriver and some way to get to the back of the alternator. I don't think you can get to it from above in a 420SEL.

In my case the dash lights lit up at random then stayed on then the car died and wouldn't start.

The only time in the 20K miles I've had the SE that it left me stranded.

Sixto
91 300SE
87 300SDL
83 300SD