The iPhone models currently sold on apple.com work with nano-SIM cards from compatible carriers*. And they work on many 4G LTE networks around the world. Contact service providers for details.
Available space is less and varies due to many factors. A standard configuration uses approximately 10GB to 12GB of space (including iOS and preinstalled apps) depending on the model and settings. Preinstalled apps use about 4GB, and you can delete these apps and restore them.
* Trade-in values vary based on the condition, year, and configuration of your trade-in device. The trade-in value offered by Apple Retail Stores and the Apple Online Store may vary. You must be at least 18 years old. In-store trade-in requires presentation of a valid, government-issued photo ID (local law may require saving this information). Additional terms from Apple or Apple’s trade-in partners may apply.