Just a note:
On newer (90's on up) vehicles, no longer will you experience the telltale "slow crank" when the starting system announces that it's on it's way out! This is for MB and any other models.
The engine management systems constantly monitor voltage, and it there is not enough juice to fire up the engine, then all that will welcome you is the distinctive "click".
When the battery in my W124 went out, I was greeted with the same "click"...started one day, "click", the next. Same experience occurred for a '91 Honda Prelude and a '90 Mazda MX-6 we had years ago...and both solved with fresh batteries.
The original should last up to 5 years, but all sorts of environmental variables can shorten battery life.
I would start with the battery...
