I have a factory unlocked white 64GB iPhone 5 running iOS 6.1.4. I bought it off eBay back in late 2014 and it was new open box. I did have to wait a few months before the version that met my criteria became available and I reached out to a lot of sellers cause I wanted to buy an iPhone 5 that met exactly what I was looking for (6.1.4, white, a1428, factory unlocked, 64gb). I did eventually find someone. My recommendation would be to subscribe for email alerts or eBay app notifications when new listings in ebay match keywords like 'iPhone 5 iOS 6', 'iPhone 5 6.1', 'iPhone 5 6.1.4.' 'iPhone 5 64gb iOS 6' etc.
I use my iPhone for the basics - music (the music app before it all went downhill), phone and browsing. If you have iTunes Match, iOS 6 works great with it. I also have a running app, a few games, podcast app (also before its downfall), utilities and entertainment apps and I use personal hotspot with ease.
That being said, using iOS 6 now will have limitations and I have had to give up on apps like Uber (and use the mobile site instead). As long as you purchase/download an app on iTunes with your Apple ID, you will almost always be able to download the latest iOS 6 version on the app store as long as it existed unless Apple changes this policy in the future.