The Mk3 does seem to have been better rust-proofed than the Mk2, but these are now 14 years old so you'll need to check each example individually. It's not just the rear arches that rot either - bonnets can rot because of stone chips, you'll also want to check for poorly repaired accident damage, and keep an eye on panels where the rubber seals have failed. The power-operated hoods can suffer from blocked drains (check for condensation) and can also get out of alignment. There is the odd horror story, so you'll want to make sure the roofs been well looked after - bolts kept tightened, joints lubricated etc. Again, thoroughly test the model that you're working on, listening for any clonks, rattles and unexpected noises.