Nivomat vs. standard shock absorbers

Nivomat shocks are noticeably thicker than standard shocks (about 80 mm vs. the narrower standard size).

Yes, but Nivomat system springs are softer because the shocks do part of the spring work. So also replace the springs. A good option is for example Sachs Advantage shocks + new springs. If you need load capacity, consider caravan tow springs.

Measure the distance between the tire and the fender on both sides with the car on level ground. Add approximately 100 kg weight to the trunk, take a short drive and measure again. Well-functioning Nivomat shocks should have little to no difference.