Rental property abroad - how do i calculate depreciation?

1/ What is the cost basis? How do i get it? 

2/ What if there is substantial time between when the property was purchased and put to service for rent (like 20 years) ? Cost of real estate is dramatically different...

3/ How do i decouple land value and building value abroad, under block house with 48 apartments? 

4/ what exchange rate is used? 

5/ Do I HAVE TO calculate and use depreciation or can i simply tax income from the property less the expenses (utilities and insurance)?