The S320 did indeed come in short or long wheelbase; the V8s
AFAIK were only offered in the long version.
What Larry said about "condition, condition, condition" is absolutely key for MB, and never more so than for the 140 series. No matter which engine it has, you'll like one that's really, truly, fully sorted out, but you and your bank can be significantly punished otherwise. Do Not make the mistake of buying the green 500 rather than the blue 420 because the color's nicer, and finding out you're the guy who gets to do the wiring harness, head gaskets and AC evaporator! The 140 can be a great car, but it can also chew up $5K faster than you can blink and then come back for the main course.