It is all about networking and growing friendships and partnerships, IMO.

Should I do signed  partnership  agreement?   

it is a 50-50% partnership between you and the tenant. 

This will put you in places like: Katy, Cypress, Tomball, Spring, Humble, Huffman.Check out the Greater Houston Partnership's website for some great information on the economy and demographics too.

Or are you in a partnership?

Landlord lived at the property for over a decade and never had such a situation. 2) A/C likely not functioning.

He came up with an idea to give me money for down payment and we make partnership to buy first house.

If you are buying for future appreciation, you cannot use the same matrix as you might use if you are buying for cash flow right now, and its usually the function of neighborhood that you choose.Just because you can cash out for "x" amount from a given property, doesn't mean you have to.

I am in turn intending to "sell" the property to a partnership LLC that I have with two other partnersI'm in need of counsel on the best way to do that. 

I'd like to know more about your partnerships or the ones you've seen other GCs set up with investors.