Macbook Pro internal hard drive issues, any way to get this working?
I would start again to see if you can test the MacBook Pro's hardware (including the internal drive). This is also a good idea anyway because of its age and the fact you don't know the history of this Mac. 

Run Apple Hardware Test

Your model MBP uses Apple Hardware Test. To use this, follow these steps:


Shut down your MacBook Pro 
Restart your MacBook Pro
Press and hold the D key before the gray startup screen appears.
After a while, Apple Hardware Test (AHT) will start.
When prompted, select your language and click the right arrow. 
When the AHT console appears, you can choose to run Basic tests by clicking the Test button. However, I suggest you select the "Perform extended testing" checkbox before you click the Test button.
Your test results will appear in the window in the bottom-right of the console.


Note 1: The extended test will take some time. Take a note of the results and report back. 

Note 2: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.

Assuming you find no problems, I would then reset the NVRAM of your Mac as well. Before doing so, unplug all external devices (including monitors, keyboard, etc).

Resetting NVRAM on your model MacBook Pro

Your Mac use Non-Volatile Random-Access Memory (NVRAM) to store a whole range of settings. Here’s how to reset this:


Shut down your Mac. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionPR keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac tries rebooting again.
Let go of the keys and see if your Mac boots up (not likely).


Assuming it doesn't boot, try again to install macOS onto the internal drive to see if it makes any difference.

Finally, if you couldn't get Apple Hardware Test to work at all, but you do now have macOS installed on an external drive, you could boot up from that and see if you can run Disk Utility to test and/or repair the internal drive. Assuming you can do this and it passes, you could then try installing macOS again onto the internal drive while you're booted from your external drive.