How long do these engines last properly maintained
Looks like it had options for 3 different engines, 3.5L V6, 5.5L V8 and a 6.3L V8, which one are you asking about??
Just doing a little Googling shows a Mercedes Benz E-Class can have a long lifespan around 250,000 mile range or more…
How Long Does a Mercedes Last? | Reliability, Longevity & Maintenance Tips.
Depends how long you can afford to properly maintain it.
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I think that is all of the engines (for that year anyway) that MB offers, I only found 3 for the E-Class US market…
E350 = 3.5L V6
E550 = 5.5L V8
E63 AMG = 6.3L V8
Looks like Mexico had an E300 and E500 with a few more engine sizes…
If this has the M272 or M273 engine . . . I wouldn’t touch them with a 10-foot pole
And why exactly did you ask how long they last “properly maintained” . . . ?!
What engine oil viscosity are you using and does it meet the proper specs?
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In case the OP thinks that db4690 is just “throwing shade”, you should know that he is a former Mercedes-Benz technician, and he has seen it all… both good and bad in the shop. I suggest that you take his opinion very seriously.
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Considering the engine is already 15 years old that’s a tough question. Do you have reason to believe the car has been properly maintained so far? And by that I don’t just mean regular oil changes. I mean oil changed using factory oil filters and Euro spec engine oil.
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I predict somewhere between 10 weeks and 10 years.
Of course, the car could get T-boned in an accident tomorrow.
It all depends on how the car has been maintained up to this point.
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Generally 12 hours after the warranty expires.
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What are the common failure modes?
I remember many cam phaser, valve train and balance shaft problems
I’ve replaced MANY cylinder heads on those engines
I did some quick “research” and it seems OP’s engine may be just past the real dark ages, but I still wouldn’t touch it
And don’t even get me started on the 722.9 automatic transmission, if this car happens to still be so equipped 
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Merc has a pretty good reputation for building quality, long lasting engines. But on this one, maybe the manufacturer’s oil & filter change interval recommendation needs to be revised downward?
Have a 2005 MB 3.2 lite V6 and currently have 180.000 with no repairs aside from slow oozing valve covers which I’ll have to take care of someday.
HOWEVER, I have been meticulous about following regular maintenance with the specific fluids and parts changes recommended by MB which points out the importance of regular maintenance.
They’re not called “Merc”
Mercedes, Mercedes-Benz, Benz, etc. . . . but not “Merc”
“Merc” is Mercury, afaik
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Not a good comparison, imo
Your 2005 E320 has one of the most reliable engine, the M112
As opposed to the far more problematic M272
I can’t tell you how many customers with an E350 with the M272 did EVERYTHING correctly and still needed cylinder heads or complete engines replaced . . . and at low mileages, btw
And some of these guys had the engines AND the automatic transmissions replaced, also at low mileages
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The OP will probably choose a different car. Many cars are flawed in one way or another. If their next choice is better than this one, I’ll attribute it to dumb luck.
Good info, tx. Just curious, What oil change interval does MB recommend for your 2005 3.2 V6? Is that what you use?
hmm … I seem to see MB referred to as Merc pretty often, but maybe that’s more common in Europe. For example
Incorrect image removed, corrected image in post 25 below.
Without checking the owner’s manual I believe it’s 5 or 6 thousand miles but I change and reset the service sensor at 5,000 miles and it’s 8 quarts Mobil 1, 0W- 40 or 50, what they call the European Blend.
Coolant is also specific but I’d have to check it.
BTW, oil and coolant changes on these older engines is incredibly easy, requiring no tools beyond a paperclip and can be easily done, with the aid of a $20 electric, boat oil pump from Bass Pro or Walmart by simply opening the hood…
And while the oil is kinda pricey, the oil filters are downright cheap, ranging from $1.22 to $7 - $8 for the Mann, Bosch or Hengst filters.
. .