The E420 210 chassis is a hell of a car but kind of an oddball.
To start with I want to know the production date or at least month.
Up to 5/96 they were DAS 2a. Starting 6/96 they were DAS X.
Only the DAS 2a cars would even have the blinking red and green lights, so if it is DAS X (6/96 or newer), you wouldn't have the blinking lights so you might have a DAS problem like you say. I just hate to see parts like a crank sensor and fuel pressure regulator thrown at a car like that, yeeesh. But yeah there is a possiblity a new key might fix it, it has a transponder in the key. BUT HEY there is also an antenna ring around the ignition lock, I know they also fail, and a DAS module and wiring and..........you get the point, figure out the problem. As I like to say, you can fix a car using the shotgun approach but the ammo is pretty expensive.
I assume you do not have a second key you can try??? That would be free to try. Free is always good.
PS never heard of "resetting" DAS, either it works or it don't.
PPS there is a black (I think it is black anyways) label in the drivers door jamb that will list the month and year of production, should be on the body part, not on the door, sort of near the latch striker. Will be in one corner of the label, just real simple like "9/96" or whenever it was made.
Here, found image of sort of what the decal i am talking about looks like, see where it says "07/93", that's what I want:
