Is Zakat due on the principal cost of purchasing a car leased to an oil company, or only on the monthly income derived from it? And is Zakat calculated on the principal cost of purchasing an apartment or a plot of land for investment purposes?

Zakat is not obligatory on the value of a car, real estate, or land if they are not for trade. Rather, it becomes obligatory on their income or revenue if it reaches the nisab (minimum threshold) and a hawl (full lunar year) has passed over it.