Gst purchase

Your query is not clear

He can purchase as unregistered person .
Yes URD cannot make Inter-state supply

The Person can purchase as Unregistered Dealer. There's no such Provisions to quote Aadhar Number in case the business is not registered under GST or something else. You can purchase from any dealer ( Registered or Unregistered / Inter State or Intra State ). The only thing is that you will not be able to enjoy the benefit of Input Tax Credit ( ITC ) under GST being an Unregistered Person. It becomes your cost.

As far as limit is concerned, there's no such Purchase limit. If your Outward Supply ( Sales ) exceeds Rs. 20 lakhs ( Rs. 40 lakhs in case of Exclusive Supply of Goods ), then you have to apply for GST Registration U/s 22 of the CGST Act 2017.

he can purchase an unregistered dealer but no need of ITC