big motor problems

[bmb300sd]

I have a 1983 300SD that was just running great except that it was really hard to start. I was driving on the freeway and when I slowed down to turn around under a bridge the car died and would not start back up. It just stopped. I took it to the MB dealership and they said it needed new glow plugs before anything else so it would start. so after $600.00 it has new glow plugs and starts right up but after it started you can hear a small low end knock. and it runs rough. I took it on a short drive under 50mph just to get it home. it go hot around 100c and then died just like last time when I got a trailer to tow it. it started right back up. the knock is bad but not the worst I heard. I also saw a lot of smoke coming from the tailpiple it looked really black so I don't know if it was unburned gas or oil. I thought these engine where "Bullet proof" so what are the odds it is just a rod bearing? if not that when what else? and can I replace a rod bering without taking out the crankshaft? I really need your help I have spent a lot of money and don't want to buy a new engine. so any ideas would be helpful