All fwd cars are more difficult in service and therefore also more expensive in repairs but the only thing (almost) that is more expensive to work with is the tranny.
Parts are probably a little more expensive also.
960 has a one cylinder longer 850 engine wth rearwheel drive and therefore a little less expensive in work with the tranny but this was volvos top of the line and the part are even more expensive than the 850.960 91 to 94 has still got the "box" like shape youre after
74x/94x is probably the more economic choice but the safety is better in the 850.
Parts for the 7-900 is cheap in sweden and you can do most of the work yourself.
850 are a little more expensive but is a smother ride and a safer one.
lots of pro:s and con:s.

the answer about closing year for thedifferent models i think is easy, they sold them as long as they could make money out of them, 240 production years 1974 - 1993, and the 740 82 - 92, ergo one year earlier than the much older 240.
91-92 they manufactured 240:s 740:s and 940:s side by side, that is more strange than 850 and 940 beeing produced side by side, after all they are different cars from the wheels up.
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V70 2,5T -98