You might have to make an actual written offer to get into the property. Take the advice of a few other posters and make sure your offer has a clause that allows you to walk away if you don't like what you see. make sure it allows you to get back any upfront money you may have to fork over.
The one document I like to get to prove property income is the seller's tax return - at least the schedule that shows the numbers from this property. To this day, i've never had a seller turn down that request. They've waited ot the last minute to get it to me, but never ultimately turned it down. I also put in my contract that the offer is conditional upon my review of those documents. I ask for tax records because, in my opinion, they are better than some excel file the owner can crank out before your meeting.
if you are buying from a larger company or investment group that will not disclose that tax document you will get the income and expense sheets from them or their management company.
make sure you confirm these numbers the best you can. Since it's your first deal you may not know what the right number are for expenses, so make sure you check back on this blog for experts who might be investing in your area. If you need help, get a savvy broker in your area who can help you.
If you think you will be getting financing to buy the property I would start looking into that process now. Some of those lenders may require documents that you might not have thought to ask for. It would be good to stipulate those docs in your offer letter as well.
If the owner can't, or won't, provide those docs it doesn't matter - you wont be able to get it financed, walk away-unless you want to buy cash or setup some other creative purchase.
Good luck.