This thread might help you with the leak issue, as the chap in there had a leak in his 940 too.
https://www.volvoforums.org.uk/showthread.php?t=281539
Regarding some of your other points, anything over 20 is a classic car and so subject to very cheap classic car insurance. A 940 TDi also shouldnt be significantly slower than a 75, unless its the V6 model. Check for boost leaks.
As to what car to keep or what to buy in replacement, that is something that ultimately only you can decide as only you know your scenarios and have first hand experience with the cars.