In mid 2012 I had a few issues with my 1997 E420 and went to driving a beautiful 1990 560SEL I had instead. I never got back to the E420 and It was left garaged and untouched this entire time.
Last year I lost the transmission in my 560 and sold it and sence then been driving my wife's Volvo wagon, uggg. Reliable good car but doesn't feel anything like a Benz. So I am now bored to death so bringing the E420 back on the road is my summer project with my daughter. I parked it for a few reasons:
When parked had 126K miles on her.

1. Cooling Fan Failed. (Replacement part on hand)
2. O2 Sensor pre cat on passenger side failure. (Replacement part on hand)
3. Misfire 1, 2, and 6. (New plugs, ignition coils, and boots on hand)
4. Radiator has small leak along seam between metal and plastic down driver side. (Have not purchased replacement yet)
5. Needs rear break job. (Parts on hand)

It was parked with a full tank of premium gas but it smells of vinegar now so clearly I need to drain it completely. I plan on also changing oil and all filters and battery before I even try to turn her over.

So other than the above repairs I was wondering what more I should do before getting her going again?

Before you spend a dime check everything for rodent damage!!!

And drain the fuel so you don't pump junk to the injectors, test the fuel pump. There is a high likely hood it will be locked up.

Thanks guys, luck for me my garage has more black snakes then anything else in it so didn't find any rodents or rodent damage yet, however it took me a day to clean all the mud dobber wasp nest off the under side.