Is zakat due in the coming years on the aforementioned land if its owner intends to cultivate it, with the possibility of selling it when needed, and how is this land to be valued for zakat purposes?

So long as your intention has settled on cultivating the land, zakat (definition) is not obligatory on its value when a year has passed, because it is not among trade goods. And your intention to sell it when needed does not change its ruling nor make it a trade good. Based on this, you are not required to search for how to appraise the land.