Hi,
1. No
2. Yes. The numbers will be different. This is reason for 1. No. And there should be no problem for that.
3. New company code - nothing special
New purchasing org - nothing special
New purchasing group - should be placed under purchasing organization. Reponsibility tab should be maintained accordingly. Meintain customizing in SPRO: "Define Objects in Backend System (Purch. Reqs, Reservations, Purch. Orders)".
Of cource maintain attributes for the new objects.
Regards
K