1. Yes, you can copy the new plant C from plant A, Then change the assignment and all required data for new plant C.
2. You need to maintain these number ranges which is under company code like as which number range is under inventory management.. But in Purchasing, no need to maintain number range because its not under company code.
3. Yes, If your client hasn't need any new pricing procedure, then it is enough to maintain old calculation schema to new purchase organization from OMFO.
4. All are important criteria, you have to looked out all of things for new company code, plant, account determination.