M-B diesels in the 123 chassis (1977 - 1985) (240D, 300D, 300D
Turbodiesel and 300TD (station wagon) are tough, durable and reliable
machines. Each has its advantages and should be sought according to YOUR
needs.

Your late interest in a diesel is to achieve fuel economy so know that
these old engines deliver MPG from mid 20s to high 20s, using diesel #2.
Biodiesel is available in Bio100 (100%) and Bio20 (20% bio, 80% diesel)
in some areas but doesn't achieve quite the mileage of diesel #2.

These old diesels require more maintenance than say, a Honda or Ford so
a DIY maintenance program IS part of the cost saving equation. If DIY
mechanic work isn't in your equation then the repair cost will overcome
the diesel's fuel cost saving. DIY includes oil and filter changes at 5K
miles, engine valve adjustments at 15K miles and (all) filter
replacement at 30K miles. The cars are not hard to work on but differ
somewhat from domestic cars so one ought to buy some manuals to do the
jobs correctly.

The 4 cylinder 240D (2.4L) is the most basic model and was sold in the
USA with 4 speed manual or automatic transmissions thru '82 MY. The 240D
engine's modest power favors a 4 speed manual transmission; the
automatic version is OK for local driving but too slow for freeway
acceleration, IMHO. If his commute is local or back roads with no long
hills a 240D is a good choice.

The 300D is basically a 240D with a fifth cylinder added to make a five
cylinder 3.0L engine that makes 77 HP in its non-turbo version and 120
HP in its Turbodiesel version (which started in '82 or '83 MY). These
cars have power windows, and automatic climate control systems, most
have M-B Tex leatherette upholstery; real leather was an option. The
Turbodiesel is clearly the preferable of the 300D as its 0 to 60
acceleration is in the 12 to 14 second area whereas the non-turbo motor
is in the 18 - 20 second area. (Most new cars' are in the 10 to 12
second area.)

The station wagon is good but only if you need a station wagon.

These cars are now 20 to 25 years old and many have done a lot of miles.
Some will tell you that M-Bs run for 500K miles - and some do - the ones
that get their oil changed every 2K miles and have fastidious owners. A
poorly maintained MB will be scrap after 200K miles so a car's condition
is the ONLY criteria to believe. When you are serious about a diesel car
spend $100 or $150 to have an independent M-B shop check the motor's
compression for that will assure you that the car is worth the $$ or
that it's not and thereby you avoid a financial hole. These cars are NOT
cheap to have repaired, DIY may become necessary. Most parts are
available on line from various vendors, parts are not cheap but are
readily had.

Finally, don't buy a rust bucket. Check behind the firewall mounted
battery, the fenders and spare tire well.

There's two broad categories here. Older and newer cars. Note that it's
reasonable to drive a fairly older car with one of these than you might
be used to. Well kept these things can last a long time, 500-700K+ miles.

I have a relative by marriage who stil drives his owned-since-new and
still-looks-new 1966 model.

For newer models, that's anything news than a 201 chassis, somebody esle will
have to fill out this part.

For older cars you basically have:

115 chassis - the old 240D
116 chasis - S class 450Se with a turbodieselmotor.
123 chassis - the new 240D, old 300D, turbo and non-turbo
201 chassis - the 190D baby benz
126 chassis - the S class 300SD turbidiesel
124 chassis - the mid size E class electroturbodieselmotor

the 126 is the last chassis that I think gives the average
mechanic a fighting chance. post 124 chasis cars have
so much electronics you'd better be a bit of a hacker to be
able to sort one out.

I'd get a 300SD but I'm biased I suppose.

Avoid all rust. DO NOT buy a car without a reputable
MB guy inspecting it period full stop.

Look at cars and ask question here.