It sounds like you've got a company car allowance deal and that's why you're in the market...?

Most gas Mercedes that I've seen require midgrade or premium. An E55 would no doubt require at least 92 octane, possibly even 94... but I could be wrong with the 94 part.

Definitely pick the car that you'll be the most happy with. Electronics doesn't always mean less reliable.

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1981 300SD - 283,000 KM's at purchase, 360k+ now. Engine replaced at 311k. 16" CLK wheels, w126 gen II cosmetic upgrades, late w126 leather interior. RIP. Parted and gone due to fire

1987 300SDL - 243K miles and counting. In winter hibernation!

2001 Ford F250 Super Duty "Platinum Edition" Lariat 4x4 7.3L turbo diesel, 295k+ miles, various mods for reliability and performance.