Assuming that you were keeping whatever business/tenants that are in there; the Seller is inlikely to share his personal/business finances with a stranger. You will have to show them that you have integrity and are a serious Buyer, before they will share with you.
If you are going to change anything; then the previous income statement will be meaningless to your situation.
In either case, you obviously will need a good commercial Realtor helping you.
That's a good question! You should work directly with an agent who can help you learn more about that specific property (and probably will know a lot about similar properties in the area). To find an agent, just click on the yellow button on the property's listing and you will be able to start a dialogue.
Please let me know if you have any questions or if I can help with anything else. I am happy to be a resource for you. Good luck with the search!
Ali, Trulia Voices Community Manager