Mercedes gas motors will run just as long as any diesel.
IMHO the best bargain now on a used Mercedes are the 2001-02 E320/430's. They are old enough to be cheap, but still new enough not to be a project. Great mileage, lots of power, easy to work on, not much goes wrong, what's not to like? The M112/M113 are nothing special by modern standards, but they run good and their isn't very much that goes wrong other than a sensor once in awhile. The 722.6 behind them is a fantastic and by that time bullet proof gearbox. My friend has close to 220k on his and its still going strong, wouldn't be surprised if it went over 300k miles. On one ATF change to at 150k!
You can even get them in AWD if you live in a snowy area.
Personally I'd get the V8 since its got more power and there really isn't a downside.
Change the transmission fluid every 100k, oil every 10k and call it good. Spark plugs are a bit expensive but they only need replacement every 5 years or 100k miles so not a huge concern.