First, it would have helped to post the YEAR of your car, since it makes a big difference!
From your profile I found it was a 1987 300TD. Given that info, you'll want to read this:
http://www.meimann.com/docs/mercedes/OM603_high_temps.pdf
http://www.meimann.com/docs/mercedes/OM603_radiator_notes.pdf
http://www.meimann.com/docs/mercedes/Citric_acid_flush.pdf
At this point I think you need to have some testing done with an infrared thermometer. You need to get the car hot then test various points on the radiator inlet & outlet, head, t-stat housing, etc to see what you find. The condenser fins must be clean. Per the three MB factory documents above, a citric acid flush is DEFINITELY in order. The electric fans are NOT intended to keep the car cool, so while it's good thing to verify that they function properly, they are not the cause of your problem. Both my 87's run at 90-95C all the time. Under load (full throttle, etc) I can make them get up to 100C but that's about it. Replace the plastic tank with the new style plastic tank ($40), which has a special anti-corrosion pack inside (there are no metal tanks made for the 124 and I don't know why you'd want one anyway).
Finally, PLEASE pull the water pump and inspect it! A friend of mine with a similar problem to yours (same car) also replaced everything and the car still ran to 120C on hot days. Check out what he found when I finally convinced him to pull the pump:
http://www.meimann.com/images/mercedes/W124_stuff/pump_impeller1.jpg
http://www.meimann.com/images/mercedes/W124_stuff/pump_impeller2.jpg
This helped his problem but hasn't totally cured it, but he hasn't done a citric acid flush yet. He did a SuperFlush with McParts cleaner and said it came out BROWN, so I told him to keep flushing with cleaner until it comes out CLEAR. He hasn't done that yet. Note that the flush procedure could possibly clog the radiator so be careful. If you have the old radiator, use that for flushing. If the radiator is more than ~2 years old, it is NOT new and may require replacement! I chased a "hot running" problem on my first '87 because the radiator was 4 years new and I thought it was fine. Nope, that was the problem, replaced that and it would rarely go above 95C in summer... keep us posted on how it's going.
BTW - with the car hot, feel the radiator surface in front of the fan & see if it's hot. It should be VERY hot (don't get burned) and should be hot over the whole radiator. Oh and don't use aftermarket t-stats (from NAPA, etc)... ONLY use an OE or OEM (Wahler, Behr) thermostat. Trust me.
Oooops, almost forgot. How do you know if the head has no cracks? They are often tiny and hairline and hard to see. If you have the old style head (603-xxx-yy-zz, where "yy" is 14 or 15) it's almost guaranteed to be cracked after a trip to the redline. You'll want a new style head from a 3.5L diesel engine ("yy" is number 17 or higher). MB redesigned the head, AND head gasket, a total of 5 times (six different part numbers). Here's photos of my cracked head:
http://www.meimann.com/images/mercedes/head_gasket/crack5.jpg
http://www.meimann.com/images/mercedes/head_gasket/crack6.jpg
HTH.