If he doesnt pay eletricity as sounds to be he can invest some money into miners new or even used,anyway,he can mine some bitcoins even roi with the current value of bitcoin ,if it raises well the roi will be faster,if he reinvest all mined coins into new equipment well he can for sure manage income and bills of equipment.
I don't agree. This reasoning is without base. The price of hardware is extremely high and it is not easy to break even and to have profit if you buy one of them. To not tell that with that profit can be able to buy new equipment (hardware to mine). This is impossible. Cannot have such profit mining with any of the mining machine in circulation (having in mind their price).
Honestly it depends on your electricity price. If someone is sitting at .15 or so.... a S7 is his only chance. It has the highest efficiency and he can run it tiil he needs to sell it. The hope is he run's it long enough that by the time he sells it that puts him into ROI.
But if cheap lets say .05 or something yes old gear will do great for your. Get some S3's or other decent gear and you will likely do good.