Buy 3.5 adapter from Samsung authorized dealer, or if you buy from Samsung distributor ask for warranty if they give warranty for 6 months and bill, that's a genuine product. For otg, a samsung otg cost around ₹300 or there are many options under 300₹ too starting from ₹60 as well in the offline shops, take a pendrive to the shop, and test otg with copying data to pendrive and then back to smartphone, that will be helpful in knowing the data transmission speed as well.
