All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Zakaah must be paid for the amount mentioned, and the fact that you saved it to buy a land for the purpose of rent does not waive Zakaah as long as you possess it.
Once you purchase the land and started to rent it, then there is no Zakaah due on its value, and you only have to pay Zakaah on the revenue it generates.
In sum, you must pay Zakaah on the amount mentioned; 2.5% as soon as a lunar year elapses from the time you possess it, and if it reaches the Nisaab (the minimum amount one must pay Zakaah on).
Allah knows best.