The entire organization of Jehovah's Witnesses is funded by voluntary donations. There is no "company" that generates money to pay salaries.
Also, most cars that a governing body member gets is either one they bought before they went to serve at a branch office, or given to them by other brothers, or bought after years of saving
As said they don't get a salary, but they do get a monthly stipend as do everyone who serves at a branch office(my brother served at the South Africa branch which is how I know). It is not a lot, but suitable for the country you live in etc.
The governing body is mostly Americans now, but they are not limited to that. In fact many of their helpers are not American (as you can hear/see from watching the monthly broadcasts)
Choosing of a governing body member is done with great prayer and consideration - no one is just chosen and it doesn't matter the type or size of donation they make etc. It is done through the guidance Jehovah's spirit
governing body function
more governing body
paid clergy (same guidelines go for all branch members including the governing body)