Buying a shop in cda sectors

Dear experts are requested to shed light on legal process to buy a shop on installment in islamabad (more precisely in zone-2.
1:- is the transfer is made through society or through builder?
2:- ownership is awarded at the time of purchase or after completion on installment?
3:- what kind of documents i need to check/require while buying?
4:- is electricity charges/connection is provided by builder or i have to do it all by myself after getting possession?
5:- what is commission of dealer in the case of shop on installment?
This is my 1st experience of dealing with shop, so your detailed answers are requested.
Thanks all in advance for your valuable feedback.
Regards

Any one could please explain the above?