Buying Flat in Unauthorised Colony

Hi Experts, I am going to buy 50 yards flat built upon an unauthorised land in New Lahore colony, Shastri Park, near SDM court ,New Delhi. The land has been bought by builders(3 parteners) last year from the owner upon agreement and GPA and they have constructed flats over it.They have shown me all chain of original GPA and agreement documents. So i have few queries in my mind before i go for buying this flat. 1.Is it okay to buy unauthorised property? 2.Is it okay to buy property based on seeing GPA chains and agreement the builder have or i should enquire into some more depth. If yes then what else should i look for to be on safer side, 3.These builders have told me that whenever registries of that area will get open, they will be doing the registry of plot first with original owner paying some pending amount to owner and then i have to go for individual registry of my flat.Is it safe? 4.What documents should i take from builder before i buy flat? 5. Till the time registry doesnt open, builders have told me that i can buy flat on agreement an they have even sell 2 flats also on teh same basis. But my concern is the agreement which is going to build is between me and builders(since they have plot agreement and GPA from original owner). So is there any possibility that owner/builders can claim his/her property if their mind change.Means they can sell same flat to different buyers as agreement is not full proof.Is it safe? Also if some thing goes wrong in future will agrement will be considered as weapon of fighting back with these builders. Thanks in Advance. Sushant