If it’s on iOS 6, don't update it! It’s much faster and more collectible on the older software.
Are you planning on using it as a for calls, or is it more of a collector's piece for the shelf? i buy iphone 4s
If it’s the original battery, it might struggle. Replacements are cheap and relatively easy to swap. If it’s on iOS 6, don't update it
Most modern apps won't work, but it makes a killer high-end iPod or a distraction-free camera. If it’s on iOS 6