A $2900 E300 with 108K miles sounds like it needs TLC... lots of TLC.

Until gsxr chimes in, I'll suggest the 92-95 400E/E420. With it's 2.4x gears, it could get mid 20s with a light steady foot. That's on premium, though. An E320 particularly the 2.8 variety with it's 2.65 gears is another option but it also takes premium. I have no appreciation for KE-Jet. It was obsolete by the '80s and MB really stretched it into the late '80s. I'd walk past a 260E and 300E and keep going.

Be careful swapping longer legs into an older Diesel. The revs might dip into the stall range in high gear and you won't realize mpg benefits.

Sixto
83 300SD