The 5 cyl turbodiesel (300's) are almost as reliable an engine as the 4-cyl. The 123's with the ACC all seem to have quirks, but I haven't had much bad luck with mine. The 6 cyl engine had head problems in some years, research that. But, having driven a few of the later 6's, they have a smoother and faster engine. I'd love one in my wagon.
The 123 is a heavier chassis than the 124. My tech says he prefers the 123 wagon to the 124: highway manners, winter driving, heavier feel. The diesel 124 wagons are hard to find, only made for 2 years I think.
The 123 has a slight edge in reliability. May not be an issue if you do all your own work. I'd stay with the 123 to avoid surprises. With two otherwise equal cars, each with no or little repair history, you are a bit safer with a 123. Otherwise find a 6-cyl with records, pick the right year(s) and go for it.
There have been a few 4-speeds (from 240's) transplanted into 300 5-cyls. The tranny from the 190 is not rated for the torque of the bigger engine, and I am not sure it bolts up at all. My guess is you have to go automatic in the 123-4 chasses. I have heard great things about the 190D, speed for one. They are small cars, and the later larger engined model seems to be a rocket. If you really want a diesel stick, that's your choice. But the MB automatics are fairly stout.
Not too many benzes disappoint. Ones you buy cheap cost you later, but they are all great cars. .
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K