Actually, the odds for your E320 making that 150K mark are really good. The M112 V-6 is turning out to be bulletproof, and in 2.6, 2.8 and 3.2L six cylinder versions, is WAY over-engineered for it's power output. I haven't heard anything about the 3.7L version, as it's new.
The 722.6 tranny has it's detractors, but overall it's also turning out to be tough as nails.
The AC system is the new variable compressor system as used in the C-Class, and it's turning out to be the most reliable AC system MB's have seen.
The soft spots are the electrics. Power gadgets are turning out to be unreliable, and could be the biggest source of headaches.
Also, the W210 cars have inferior suspension bushings, but the V-8 cars are more troublesome than V-6's. Also, keeping the stock wheel-tire combos help preserve the suspension bits.
Now, all that said, we sold our 1998 E300 due to it being a terribly unreliable car. But, the diesel cars seem MUCH more trouble prone that the gas cars. Most of the trouble we had was engine related.
There are some reasons that the MB might be not too tough onthe pocket book long term --
-- timing chains on the M112 are double row and should easily last that 150K.
-- The body structure of the W210 is not as sturdy as the W124, but better than most every other car. Out W210 car was tight and rattle free.
-- The performance capablity of the E320 is far below the chassis design. You're not stressing the car terribly.
I've met a few W210 owners with seriously high odo readings, and all seem pretty certain the car will last much longer...
1998 C230 "Black Betty"