Francis, yup you're right on the plastic bit being a large fuse. If it blew, it was to protect your engine, rather than contain the heat and not be able to transfer the heat out of the system properly. I have a Nissens on mine due to financial constraints back then (long story), it's been great so far and we've taken the car to San Francisco and back with a full cabin and trunk, no problems overheating. Then again it was also a bit cool. However, recently it's been hot and temps soared to around 36 C, which is hotter than it is there I heard. Cooling system remained calm, same gauge reading as before.
While you're at it palitan mo na rin yung thermostat, P500 lang naman siya sa Delodur if memory serves. Cheap insurance, alongside the new cap. Delodur has both OE with MB star at 140, or alternative mo is the OEM but no MB star, also rated at 140. With a new radiator kasi it'll hold pressure much better so the cooling system will find the weakest link and possibly ruin whatever is the weakest/most worn part, once you break-in your new radiator/cooling system. So if you have hoses that look bad, look into replacing them na rin. Don't forget to drain the engine block too, you get most of the bad stuff out from the cooling system/water jackets that way. On the M102/M103 I think it's on the passenger side below the exhaust manifolds. It's a large nut/bolt I believe. When in doubt ask your MB mechanic to be safe.
Then fill up with 50/50 mixture of MB coolant, the good stuff. It'll make your car happy, and you'll have peace of mind knowing the cooling system will last another decade or more. By the way check all hoses too, don't overtighten the clamps especially at the new radiator neck/s! Torque them to spec. Rule of thumb daw dyan is if you'll keep the radiator but just replace the hoses, just cut the hoses if the clamps are stubborn. That way you won't risk breaking the necks. But if you're replacing the radiator, by all means de-clamp away
I had a friend who frequented the old board, he got a W123 240D Behr radiator for 9500 daw, sa "enkat trading" if that makes any sense (I don't know the place). 8000 is even better if Francis found one that price!