Buying a car in Chennai

Please let me explain my case. Me and my wife are from Germany and staying for one year in India on a Tourist Visa. I know we are not really Expats, but I would be very grateful if someone could still help us.
Until now we explored beautiful India with public transportation but from now on, we would love to travel on through India and beyond with our own little van. Our dream is to convert a Maruti Eeco into a camper van and drive it back to Germany. Together with our street dog Arthemis, with wich we fell in love here at the dog shelter.
For that we need to registrate the car. Our own research gave us no complete answer on how this procedure is done correctly. We’ve already visited a car showroom and they assured us they would guide us through the registration procedure (insurance, life time tax, NOC, etc.), but what we need is a proof of address. I have no PAN Card or any other contract which proofs my address, and just my passport and the VISA to proof my ID. The house where we are staying until December belongs to a friend, so would it be sufficient to have an address proof affidavit or maybe a copy of a Registration certificate issued by Foreigners Registration office showing my Indian address would work for this purpose?
We also got in contact with our embassy and they said it might be possible to register the car on our home address in Germany with an official consular writing, that proofs our permanent address, but we should confirm this with the RTO first. So far the RTO didn't answered our mails and our friend said to go there in person would be a waste of time.
May be someone has expereances with registrating a car in India and can give us some addvice. In case there is another way to get an address proof for a non-Indian, of which I’ve not thought yet?