It is obvious that you need pads and rotors to start. There is little to no duplicate labor between the pads and a steering rack in that car so the money spent will be the same whether you piece meal it or do it all at once. In reality though your rack is probably just fine and all you need are a set of rotors, hang new pads though too while you are in there. Brake rotor can be strange beasts for sure, as they heat up and cool down they can and will do odd things. I have driven plenty of Volvos that were just like yours. The rotors tend to develop a thickness variation that is only obvious under certain conditions. I have never seen a rack cause a shimmy problem, seen them bind, leak, not work at all, but never a shimmy because of a bad rack. That does not mean it isn't possible, just means that I have not seen one in 20 years of working on this product.

mark