The first rule of buying older used cars says that condition and history are more important than anything else. A good 320 will be a lot better than a bad 500.

Post 06/98 cars have a significantly higher specification, upgrades to the brakes (bigger at the front) and suspension (ever so slightly stiffer), and the latest M112 and M113 3-valve engines.

The M119 and M113 V8s are both great engines. The earlier engine is more expensive to repair and slightly more powerful (but significantly heavier). In both cases, only extreme bad luck or extreme bad maintenance will give rise to repairs. MB has never had to replace either engine under warranty. you are unlikely to notice any difference in performance. The M113 may give better (or less terrible) fuel consumption, but it will be marginal and if you're worried about fuel consumption stay away from the SL altogether.

The 500 is faster than the 320 but that's about the only difference, and that said the 320 is plenty fast; heck there was a 280 in Europe and that was regarded as adequate. Fuel consumption is similar in each case: the car weighs the same and the 320 has to work a lot harder to push it around. No doubt the 320 is easier and cheaper to work on, but you are unlikely to have to work on it unless you are unlucky or it has been neglected.

The 500 can be cheaper to buy because the market is put off by fear of high fuel consumption and high insurance costs. As I have said, the reality is that both are very similar.

It might be said that the 320 is more faithful to the origins and history of the SL: it is a 3 litre (approximately) straight-six, just like the gullwing.

I would worry more about the hood mechanism, the a/c system and the auxilliary electronics: this is where the $$$ repairs will be. Find a good one and buy it and drive it. I haven't regretted it--ever.

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