The radiator will be hard to locate as it is MUCH different from the USA version radiator. It is similar to the radiator in the Pre-126 model full size cars and may even interchange. The Euro cars DO NOT USE a recovery bottle. The radiator is made like and old Ford from the 60's. The thermostat is likely the cause of the problem. Being this is a Federalized Euro car as I gathered from your other post, the timing may have been fiddled with to get the car to meet emission standards and could also be causing heat related issues. I would check the timing and have it set back to European specs and get rid of the Federal crap if you can legally do so. My 280SE with no emissions is real fun to drive. Actually second guessing selling it it is so much fun.

Good luck,
Aaron