You are missing a crucial point. You are trying to obtain bitcoins (for investment purposes). There are two ways to obtain bitcoins -- mine or buy. In the current environment, it is generally cheaper to buy bitcoins than to mine them.

Because of the rapidly increasing difficulty you can't consider mining equipment as incoming generating capital. More than half of the income will be earned in the first month. Mining equipment should be considered to be a resource that is consumed much like mine or an oil field.