After some messing around I was able to get my soft-bricked Samsung Galaxy A5 Duos (SM-A5000) running again:
Can I recover a soft-bricked Galaxy A5 without voiding the warranty?
However, the version of the firmware I flashed (Kit-kat) didn't have Google Play on it. I tried installing the latest APK that I could find, but when I try to open the installed app is just force-closes with this message:
Unfortunately, Google Play Store has stopped.
I thought perhaps I was missing Google Play Services so I tried installing that too (from this link), but it died with the same error as above. I also tried installing version 5.5.8 of Google Play Store (after removing all previously installed APKs) with the same outcome.
Eventually I concluded that I must need to install a GApps package.
Is there a specific version I need to be installing to run with my version of Android? What are the prerequisites for this to run? And why doesn't my version already have it? I thought I was installing the stock OS?? (To clarify, the version of the OS that the phone shipped with had the Google Play store)