Car is coming up for 14 years old, so financially it’s probably worth more to you than it’s actually worth. If you traded in, it’s not likely to be a retail car, and would probably go to auction.
The advisories are related to multi link suspension arm or bushes. If you’re replacing one on one side side, then it makes sense to do the other side too. This includes front and rear arms. Audi use Lemforder or Meyle HD, anything else will wear prematurely in my opinion. Decent arms can be anywhere from £50-£150 each plus fitting. In addition you’ll need a tracking check, say £80-£100.
Audi are known for subframe issues. The subframe is powder coated, so road dirt, salt and water start to corrode on damaged surfaces. You can’t fully sort it unless you remove the subframe and get it powder coated, that’s unrealistic. What you can do, is get the underside of the car jet washed, and dried, wire brush bad areas, potentially apply rust converter then apply wax-oil or Lanoguard. That should keep corrosion at bay for another 2-3 years before you need to reapply. Expect to pay £300-400.