yes its a matter of concern if you can see the smoke get the car there is something wrong the engine oil that is getting burnt. or there is a leak somewhere.
what you can do is check if the starter of the car is working correctly, see if the battery is working and so is the alternator. and if all is good then the fuel system and filters have to be checked.
The white powder around the battery terminals is usually lead sulfate
it is toxic of its inhaled, ingested and if it gets in skin contact is very harmful.
when the battery has failed and needs changing this happens.
V8 Touareg is more than enough for everything you do on road, and for ocassional offroading.
If thinking of regular ofraoding, then spend some more and protect with skid plates and bash plate in front and give 2 inch lift.
You should see another mechanic, rather VW specialist like Capital Auto Garge. They are very good in VW. If its turbo or engine, its an important and serious issue so atleast get the right advise and diagnostic with VAG diagnostic tools.
the bedding of the doors that is a rubber lining may be off due to age, weather or any reason what so ever this can be contributing to the hissing noise.