Firstly, If the aggregate turnover exceeds threshold limit of 20 L, then assesses is required to be registered under gst. Here turnover includes exempt supplies also. So if turnover is below 20 L, assesses need not register under gst. If turnover exceeds 20 L but less than 50L then composition scheme can also be opted. Secondly, there is no connection b/w interstate purchase and gst registration. It is interstate supply that makes registration compulsory irrespective of the limit.