This should give you a good starting point for research: http://www.brickboard.com/FAQ/700-900/BuyingUsed7xx.htm

The 960 became the s90/ v90 in later years, not sure of the last year made, but you should find it in the faq's.940/960 Structurally the same cars, differences are the engine , trans on 960 has weather traction assist . most or all the later940/960 models have locking differential( very nice in snow and ice, go anywhere with snow tires)
960 engine is a bit more maintnence intensive and timing belt failures (usually due to lax maintnence) are catastrophic.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '82 Mercedes 300SD