There are a few things to look for on these Touaregs. The drive shaft centre bearings let go every few years, and unless you DIY its a replacement shaft which isnt cheap. make sure you check that on any Touareg you buy (the factory shafts have a date marked on the centre bearing).

When you test drive, make sure you test how it downshifts through the gears. The transmission can develop issues which result in hard downshifts, ie clunks or shifts rough 5th to 4th.

Obviously make sure all the electrics work. The DPF can become clogged with inner city driving like most modern turbo diesels.

From memory those are the main things to look out for.