Can iOS of an iphone sold and used with Tracfone service be updated to latest iOS?

1. iPhone 6S supports iOS 12, but we can't be sure about iOS 13. Rumor has it that they will drop 5S, 6 and 6S.
2. The internal storage of an iPhone cant be expanded in any way. But you can purchase an iCloud Storage plan, subscribe to Apple Music and store your photos, videos and music in the cloud.

NewToApplefor2ndTime wrote:

After I purchase, can the iOS be updated/upgraded to the latest version

Yes. iPhone 6S can run the current iOS 12.

and if newer versions come out later, updated again?

Unknown. Possibly. But no way to know until Apple releases the new version and states which devices it can be run on. In general Apple offers 5 continuous years of iOS updates for their devices after release. The iPhone 6s was released in 2015. So its very likely it will be able to update to iOS 13 in the fall, and maybe iOS 14 next year. Beyond that however, its unlikely to get any further updates.

If you want to guarantee more years of updates and budget allows, try a slightly newer iPhone 7 or 8 instead. Same shape and size as the iPhone 6s, but will be supported for longer.

And the phone comes with 32 GB of memory. Is that enough if I want to use it to take occasional photos or videos, and if not, can the memory be increased? Thanks for any insights, suggestions, etc.

32GB is a bit tight these days. Even with Apple's new High efficiency Photo format. Videos can also take up a lot of space. I would try to get 64GB if possible. And no, the storage space cannot be changed on an iPhone. You keep what you buy it with. There's also no SD card slot for added storage unlike most Androids.