As to the specific question asked, I would replace the rubber at the top of the shock, both top and bottom spring perches, the differential bushing, both sets of trailing arm bushings and the subframe bushings.
Note that there are two spring mounts; the botom US mount is thicker than the European mount. So if you have a stock US setup in your car, you can achieve a little lowering with the Euro spring perches. Carl Nelson springs will take you lower yet. My car sits on the Euro spring perches with Carl's springs.
Not sure if they still sell their stuff, but Hardy and Beck used to have a set of lowering springs and Bilsteins that they have specially valved to their specification by Bilstein. I'm pretty sure they can still order the Bilsteins even if they have distincontinued their springs.
As to the urethane bushings, I am not a fan. In more modern BMWs with much more robust structures, harder bushings can increase performance with little risk. The same cannot be said of our cars. Stiffer urethane bushings transmit more noise, harshness and vibration into the body of the car. This increases the stresses on the metal structure in the car, which can lead to fatigue and failure (and Karmann did not use the best steels in our cars). You can find posts here from folks with differential mount failures (hence the reinforcement pictured above) and shock tower failures in the rear of their cars. I wouldn't risk it, especially with the condition of the roads where you live. Stick with the rubber subframe bushings.
Lastly, my car is on the lift right now, and I am in the middle of removing the subframe from my car with the Ireland Engineering camber and toe adjustments added to them. I have had them loosen up on me several times. I've replaced the nuts a couple of times, changed to longer bolts with added nuts on the end of the square nuts they provide, and still they don't hold together. This design is not robust enough; save yourself the trouble.
