I have owned 3 300SE models. They have all been very reliable, and I am very pleased with the performance.
Maybe it's my luck, but in two of the models I've owned I have had to replace the water pump which was quite expensive.
My dad has owned a 420SEL and was very pleased with it. He owns a 500SE now. If I had more money I would go for a bigger engine, but fuel prices in the UK are stupid, so I will stick with the 300.
Whatever model you choose, you won't pe dissapointed!

Cheers

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Oliver