Equity Investment and partner agreement

[Kyle Brinks]

Hello,

I have worked out a deal with my brother-in-law to purchase an equity stake in his current residence and turning that into partnered rental property with him. This equity investment will allow him and his family to afford a down payment on a new home that fits their needs. We both like the deal and consider it a win-win.

I am looking for advise on what needs to happen to make this agreement legal, what to include in a contract, and what to consider when determining a proper investment price by me. 

Also, what are the tax implications of me giving him a substantial amount of money in the form of an investment.