The size of car would be a good indicator of where we could further point you MS, but I can offer the following on the Midsize 'E-class' sizeof cars.

W123 chassis 240D, 300D, 300CD (coupe), 300TD (wagon) - ('77-'85 - the 300D models were also available with Turbo from I believe '81 and on) - arguably the most durable diesel MB every built. Many, many cars with 300Kmi+ and going strong. No computers - or very few, simple and inexpensive to work on (except for the ACC systems - which can be a caveat on any other Benz's recommended here). No airbags.

W124 chassis ('86-'95 - Turbo models in the early 90's) - much more modern and civilized - although sigifigantly more complex in terms of computers/electricals. Much higher performance although the new generation of diesel engines could be problematic due the the aluminum heads mated to cast-iron blocks. owners experiences from what I've read are "just as good as the old all-iron motors' to 'an expensive headache". Hard to know if warpage/head problems were from design or lax maintenance.
Driver's side airbag from '86 to I believe about '91-'92 when the passenger-side was an option (driver and passenger airbags standard on 93+ models).

Kinda comes down to budget and what you're willing to live with in terms of cost and refinement. I have an example of each (though my W124's gas V8 is no comparison). The chassis dynamics of the W124 are superior, but I find the W123 to still be very comfortable and pleasing to drive. The quality of materials in both is very high, thought the W123 has a heavier 'old-world' feel (like it will go a million miles).

Hope that helps get you started - other will provide myriad other choices/considerations, so good luck - and search the archives to for hours of reading of owner's experiences.

Regards,
- Ryan

__________________
'95 E420 - 'Shadowfax' 138kmi.
'92 Volvo 740Turbo Bard 193kmi
'74 240D - 'Ol' Green' 4spd Manual 104kmi. (sold )
'77 300D - 'Red' 223kmi (sold)
'75 240D - 'Bianca the Blue Bomber' (sold)