I used the same MasterCool AC ferrule crimper when I re-hosed the oil cooler lines on my 1984 300D. I used AC barrier hose, which is rated for hot oil and much higher pressure. No leaks in ~5 years. The OE hose had been hacksawed by the p.s. belt, due to sagged motor mounts. I cut the new upper hose 1" longer to better clear that belt. Amazing M-B never thought of that.

I'm surprised that a 240D even needs an external oil cooler since even classic V-8 U.S. engines don't. I assumed it was needed for the turbo engines since they have oil squirter coolers under the pistons. My 2002 Chrysler 3.8L minivan has an oil cooler, a simple pancake between filter and block which routes thru engine coolant, so your first idea of using the radiator transmission cooler "should work". Ours has the factory tow-package and AWD, which adds that oil cooler and p.s. cooler.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans