Post '85 your choices are:

86-89 190D 2.5 = W201 2.5 liter 5 cyl 602 optional MT
87 190D 2.5 turbo = W201 2.5 liter 5 cyl 602 turbo
86-87 300SDL = W126 3 liter 6 cyl 603 turbo
87 300D or TD = W124 3 liter 6 cyl 603 turbo
90-93 300D 2.5 = W124 2.5 liter 6 cyl 602 turbo
90-91 350SDL = W126 3.5 liter 6 cyl 603 turbo
91 350SD = W126 3.5 liter 6 cyl 603 turbo
92-95 300SD/S350 = W140 3.5 liter 6 cyl 603 turbo
95 E300 = W124 3 liter 6 cyl 606
96-97 E300 = W210 3 liter 6 cyl 606
98-99 E300 = W210 3 liter 6 cyl 606 turbo

If there's an engine to avoid it's the 3.5 liter 603 because far too many suffer from bent rods causing oil consumption and smoke. They are not unreliable since they will go a long way before oil consumption makes one impractical. If you're a DIYer, a 3.0 conversion is pretty easy and good used 3.0 blocks are available. Take a good look at a 3.5 with a factory replacement engine. Few factory replacement engines have recurring problems.

The W140 can be a money pit with all the little and not so little things that can go wrong such as the AC evaporator and closing assist. A poorly maintained one is costly to keep on the road if you're not a hardcore DIYer.

About the only caution with the 95 E300 is stuck glow plugs. Stuck glow plugs can cost you more than "AC just needs a charge." If I were buying one from a dealer, I'd have the glow plugs replaced before taking delivery.

Sixto
87 300D