Calculating cost of acquisition using cost inflation index for commercial property

I had bought a commercial property which was bought in installments starting from 1996 till 2004. Subsequently registration payment was made in 2013. The commercial property was sold in 2018. My question is regarding the Cost of acquisition calculation using cost inflation index.

1.) Is it applicable in this case or is it only for residential property?

Yes  LTCG can be calculated based on indexation in this case.

2.) Do I have to find fair market value of each payment separately and then add them up or do I take the year in which final payment was made and consider that as the year of purchase?

Depends upon date of possession received.

3.) Is cost of acquisition calculated taking the year in which the registration was done?

Mostly from the date of possession.