question about 3 liter 1984 turbo diesel 5 cyl
 

I am looking to buy a MB that has the 3 liter turbo diesel 5 cylinder. Is this one of the more reliable motors?

the most reliable out of all the 61x series is probably the 616 in the 240D... but if you want to be able to beat the schoolbus off the line the om617(turbo) which is reliable will be your best bet, its all a mater of prevenative maintence and keeping watch on everything that makes anything reliable imho

Forgive me for my ignorance, but which motors are which? I am not totally familiar with the engine codes just yet.

The 617 is a five cylinder engine and was put in the 300D(sedan), 300CD(Coupe), and 300TD(Wagon) starting in 77 or 78 depending on the model, and continuing until 85. All of these got turbos in or around 81.
The 616 is what is in the 240D, which is a 4 cylinder engine.
These are all the w123 style MB's, which are great, durable, riliable cars, IF taken car of.
This website has some good info.
http://home.hiwaay.net/~gbf/mbmodels.html

There r 7 engines that are recurrently discussed in this forum:

Om615= 2.2Liter 4 cylinders
Om616=2.4liters 4 cylinders
Om617= 3 liters 5cylinders
om601= 2.0L;4 cylinders
Om602= 2.5L;5cylinders
om603=3.0L;6cylinders
om606=3.0L;6cylinders

om601 is 2.2L, BTW and there is a 3.5L version of the OM603 used 1991+ in the 300SDL, 300SD, and 350SD.