Hello,
I am from Africa and have a source of free electricity.
What hardware is recommended for me? A used S7 will do? How much can S7 profit under present condition?
Also can Antminer can mine other curenncy?
The antminer the S7 can only mine coins with the sha256 algorithm.
It all depends on where your free power is. The s7's and s9/t9's are very noisy (something like a loud 1500w hair dryer on full blast all the time) and produce a lot of heat (about the same as 1500w space heater). Decide if you can tolerate that in the location where you have free power. Just use a mining calculator
https://bitcoinwisdom.com/bitcoin/calculator and plug in your variables to figure out how long it will take you to achieve ROI.