Hi Rodrego,
Most questions you asked need a lots of assumptions to be made by anybody to respond. It would be good if you could do the following, if you will.
1. You should include more details about the actual solution of Ariba and SAP your are referring to. Especially with Ariba you have to mention if you have chosen for a Source to Pay or Procure to Pay or Source to Contract or just Invoicing etc.
2. Give some background about the data that you have and looking to migrate.
3. Your question should be split in at-least 6-8 different questions so its easier to respond and state assumptions
4. For each split question you should provide details on the level of/key customizations you have made in Ariba On-Premise along with on-premise version that you currently on.
5. Try to include as much information on questions you have on as-is state of your transactions related to that question.
Some questions I can try and answer for you:
Answer to Question 2: Ariba treats material or service as an item an all items can be received
Answer to Question 7. Ariba provides reporting across all transactions that happen in Ariba and you need not use other tool if reporting is limited to transactions in Ariba. Also there is an option to import external data and referred to as Data Enrichment and it will have its advantages and challenges.
Answer to Question 8. Yes, contracts can be bulk loaded, requisitions can be imported etc.
Answer to Question 9. PO Range can be can be configured in Ariba
Hope this helps
Regards
Vilas