500 is often cheaper than 320 on the used market because of the perceived higher running costs of the 500. In fact, insurance is the same and the 500 gets equivalent, sometimes better, mileage than the 320 because the gearing is different and the engine works less. These issues were well-discussed in a post by "Learoy", who has just bought a nice 500SL. if you do a search on that member, you'll find the post straight away.

I would go for a 500, and I would take the time and trouble to find a good one: it took me three months.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190