queries with respect to GST

We are Service Provider (providing Technical inspection and certification agency service) and we would like to know answer for the following queries:

1. Second hand car purchased for business purpose from the owner(seller) of the car who was using it for personal purpose -- Whether GST under Reverse Charge Mechanism apply?, if yes then at what rate?
2. Under GST, a service provider is required to pay GST on issue of invoice to the service receiver or on receipt basis?, till now the service provider was paying on the basis of receipt as per POT rules since the previous year turnover does not exceed Rs. 50 lakhs.
3. Under Service tax Nil Return is required to be filed in case of no service is provided or any advance received by the service provider, whether any same provision is there in GST to file NIL return in order to be GST compliant?
4. Under GST service provider is required to bifurcate the GST under SGST and CGST if he is providing within the State (i.e Intra State), but if he provide the service outside the state (i.e Inter State) then how will be the bifurcation done and at what rate?
5. What are the Return a service provider is required to filed under GST?
6. What is the HSN code for providing Technical inspection and certification agency service or we have to used the earlier SAC code?

Thanks in advance.