What a coincidence - I was just going to post about this kit when I found this thread.
I just bought this kit; it's labeled as "Bosch Duraterm Mercedes-Benz Fast glow kit for Diesel cars". It was $88. The kit is Bosch part number 0 250 201 953
-710. It includes a new glow plug controller relay and four glow plugs. If you have a 5-cyl engine, you will have to buy an extra glow plug separately. The plugs that come with the kit are Bosch 0 250 201 039
-35T. The kit is listed as being compatible with the W123D, W124D 4/5 cylinder (up to 01.89), the W126D (USA) and the W201D (up to 01.89).
The box advertises "Smooth idle, less smoke by post glowing". I wanted to give it a try because although my car always starts fine and the existing glow plug system works, the car idles rough and smokes a bit for ~15 seconds after starting when the engine is cold (below freezing). Even if I let the glow plugs run longer prior to starting, the roughness and smoking doesn't completely go away - I was hoping that the post-glowing feature (i.e. the plugs continue to run for a short period after the engine starts) would address this.
The relay takes about 30 seconds to install. It's really really easy. After installing the relay (but not the new glow plugs), I turned the key and started the car the instant the glow plug light went out. It was about 35 degrees F at the time. Instead of the usual smoke and roughness, the engine was perfectly smooth and there was no smoke!!! The post-glowing definitely makes a substantial difference. I would highly recommend this kit to anyone. I didn't even use the new glow plugs that came with the kit; I'm going to save them until my current ones fail. The post-glow relay is a definite upgrade. Buy it!
(I think Bosch should send me a big fat cheque for writing this post )