Whilst many people do put on 18 or even 19 inch wheels on their E34's this is essentailly an error. E34's were designed to accept a maximum 17 inch wheel and after that, as you have found, all sorts of unpleasant things start to happen. One of the major consequences is that the stress placed on suspension components induce rapid wear on suspension components.
Two things. Suggest a 17 inch wheel as a start. Next use Lemforder parts to replace all the stressed components. Also consider putting back the original wheels to see what happens.
If putting on 17 inch wheels restores the car's balance and handling, then only replace the suspension components as you need to. If you believe the car isn't handling well after the new wheels, then go ahead. Use BMW or Lemforder parts. They are the same but having a BMW badge will increase the cost. Hire a hoist for an hour and take a bog breaker bar to get all the bits off. Remember, the car needs to be at full rest before you tighten all those new bits up. That is, the car must be sitting on its wheels before tightening up to the specificed torque amounts. If you don't have a Bentley's get one. It has all this data included. Cheap help.
Good luck,
Geoff
Sydney, Australia
'94 525iA