Purchasing a re-sale flat in noida

Hi,

I am planning to buy an under-construction flat in sector 143. This is a resale flat. i.e I am buying this from 2nd owner.

Could you please guide me, what all legalities should I consider before buying this.

Also, shall I seek a lawyer before I go ahead with the deal.

P.S:- I am buying this flat from known sources.

Thanks

Lucky.