There's nothing to be confused by, the convention is extremely simple. The SI unit symbol for kilogram is kg, so we would always write 2kg. It's the same for other SI symbols. The SI unit for metre is 'm' so we write 100m and not 100ms (100ms = 100 milliseconds so it is important that you abide by this convention.)

2kg is correct. But it is not uncommon to see people write "2kgs"

2kg

Good question! I think it should be 2kg but I am sure I am guilty of writing 2kgs sometimes!