Composition under gst

Yes in gst U can purchase goods from any state but u can't make interstate sale.

And how are we going to show our existing stock?

Whatever you purchased till 30th June ull get credit bcoz in July u r filing regular vat return. If there is refund it ll be carry forward to gst. From 1st July whatever u buy ull not get ITC if you opt for composite scheme. & within 90 days from the appointed day (July 1st)u ve to file Gst TRN 1 FOR DETAIL OF Stock held as on 30th June