Please show a picture of what this engine is, is it the 6 cyl DOHC 24V OM606 diesel that was used in the E300D late models or is it the 6 cyl SOHC turbodiesel?
The OM606 does not have head cracking issues at all. And if you wore it out in 122,000 kms then you should be really proud of yourself to do that because I have personally seen these engines at 850,000 kms and still with 'perfect' compression and power.
If you hear a "clacking" sound at first start then either one of your glow plugs is bad or you have a bad injector. Also make sure that there are no air bubbles in the plastic fuel lines near the fuel filter, if they are brown with age then replace them with a new set from mercedes. If they are not dark brown yet then you can carefully remove each line and replace the O rings on them. I can give you the sizes if you want.
There is another point of noise in this engine, the belt tensioner. That thing will make a rattling noise at idle speed if its worn out. If you do decide to replace it then first isolate the noise, it maybe the shock absorber of it slapping on the head - easy and cheap fix. if the shock is worn out or the tensioner arm is worn out, then its expensive.
The only parts that work are genuine MB parts, the arm, pin, dust cover, dust seal is about 200 euro, the shock is about 40 euro. If you buy chinese or non-genuine benz parts from any other brand, you will hear the noise again in 200 kms
btw - if your mechanic thinks that sleeves, rings and pistons are worn out then he's mighty mistaken as this engine has short stroke (less sideways push on pistons) and the cross hatch marks on the cylinders are still intact after 250,000 kms of service (I have seen it personally)
Find a qualified technician to work on this engine, if you end up opening it, you need to carefully measure all the head bolts and replace any that are elongated beyond use, you will also need some special tools like a 100mm long bi hex driver tool to undo the head bolts and need to make a brace to loosen the crank pulley mount. Its on there incredibly tight.