Buying 12 single family homes

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I'm looking to buy 12 single family homes from a friend of the family. Looking for suggestions on how to finance this deal.I cant get a bank loan on my own, but I have 80k to put down myself and a guy willing to co-sign and cover the remainder of the down payment. If I went that rout, what would I offer this gentleman in return for co-signing and what percentage should I give him on top of whatever he puts in? If I don't go this rout what are my other options? Any and all comments are much appreciated thank you and God bless!

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What percentage  would I have to come up with? If I can't get the money on my own what are my options to offer any investors that I may find?