Did your car come with Nivomats as standard when delivered? Nivos work with quite soft springs. That means that if you replace them with normal shocks you will get a soft rear end with some sag even when not loaded. I don't know what springs can be spec'd for your car as far as those from the Volvo catalogue is concerned. There is possibly standard to go with normal shocks plus 2 stiffness ratings for HD springs. New Nivos are likely to be very expensive but best if you can afford it and are going to keep what is now quite a high mileage car. The rear IPD arb won't help you much. Don't buy cheap cheap arb drop links. They are known to be short lived. A few thousand miles or 1 year quite usual. Good ones will be at least twice the price