For doing further process it is compulsory to to have rl. No you should not buy that that kind of plots.For making further documents you may face the problem in future.
RL stands for Reduce Level. Suppose, you have 1600 sq-ft. plot but you have to construct the house or flats within 1400 sq-ft. 200 sq-ft you have to leave to leave for the road and the drainage system. But registration will be made for 1600 sq-ft.
I am agree with Saurav. An important factor when planning for drainage or road is knowing the rise or fall of the allotment that you are preparing for. This will allow you to calculate the heights or depths of drain and excavations to enable the flow of water to the legal drainage outlets.