How is the Zakat calculated for the sale of an investment building that was constructed with the intention of renting it out, and then the intention changed to selling it, given that there are saved funds, borrowed funds, and car debts?

If you intend to rent out the building, Zakat is not obligatory on it. Rather, it becomes obligatory on the rent if a year passes over it and it reaches the Nisab. However, if you intended to trade with it during construction, it is considered a trade commodity, and Zakat is obligatory on it by valuing it at its purchase price. The same applies to the second building if you intended it for trade during its construction.