cap gain tax if not 100% of the proceeds go into another house

[Ingrid B.]

Greetings. I own a property that I am planning to sell, it will have long term cap gain. I don't certainly want to invest all 100 % of the proceeds into another property. Say only 50% of the proceeds goes into another investment house. Would the taxable part be then the other half plus "half" of the depreciation? Or are we still responsible on paying the cap gain tax on half of that proceeds plus "full" depreciation?

When we purchased this house few yrs ago we put 20% down. Assuming we want to just get that original down payment back after closing, is it taxable? 

Lastly, I am also considering to buy flipper house. Does 1031 allow investor to buy and sell homes < 1 yr? What's the tax treatment?

Thank you.