Specs for Battery 2000 E320

Damsel in Distress Cold start two times quick start up immediate shut off 3rd time ok. Very limited budget got laid off. Need to make right decision. Drove immediately to mechanic don't know him  don't know if I can trust him which is always an issue with mechanics he does have diagnostic machine tho. He's been busy  so I did Google search and luckily:)) found this site, Yay! Read the existing applicable threads which stated  that the computer  on start up checks the battery and the battery is not up to Snuff  it turns itself off after starting and of all the various replies I've decided that it clearly must be the battery.  Had the same issue couple months ago  took the battery to AutoZone to get it  charged  more AutoZone says it's the original battery  another AutoZone said it's more than 3 years old  after I left it doesn't make sense what they said  it's either the original or it's either 3 years old!!!!! how to find out which is which??  At that time, called a Mercedes Benz dealer consultant. I gave him overall performance history. He said the battery would be the thing to let me down n to replace it immediately  today I finally have the money. New alternator 2 years ago. Mercedes rep also warned me that AutoZone batteries are old to make sure the battery is brand new not sitting there for more than one month he said most sit there for 6 months and so do NOT EVER! buy them but he said when I am ready to buy to ask someone who knows exactly what the specs are and make sure whatever battery I buy probably Duralast will have the exact same specs as is required I will do this today as soon as I get an answer cz I have no peace of mind worried about a no start at this moment. Bottom line I'm asking what are the specs and stamped information I should look for on a new battery and what would be stamped on the current battery to know if it is original. Maybe the AutoZone rep meant that  it was the same brand  and came from the auto dealer like an original  because no battery last for 20 years how can that be??!!