The 512 GB tier of the iPhone Air on T-Mobile. Starting from $31.25/mo.

The 512 GB variant of the iPhone Air starts at $31.25/mo on T-Mobile via Installments over 24 months. Outright purchase pricing varies; see T-Mobile for the full quote.
512 GB is comfortable for most users — about 30,000 photos, ~50 high-quality apps with media, plus several hours of 4K video. Heavy ProRes / RAW shooters and offline-music collectors should consider 512 GB or higher; iPhone Air doesn't expand via SD card.
The 256 GB tier on T-Mobile typically runs a few dollars more per month than 512 GB on the same plan. Check the storage-tier table above for the live monthly delta.
No — iPhone Air doesn't support expandable storage, and Apple/Samsung don't offer post-purchase storage upgrades. Pick the tier that fits your two-year plan.
T-Mobile offers trade-in toward new iPhone Air purchases — credit varies by your old phone's condition and any active promo. Higher storage tiers typically receive proportionally larger credits.