You should opt for at least a 64GB iPhone 6s if you plan on shooting 4K video
Refurbished Apple iPhone 6S Plus (Rose Gold, 64GB) - (Unlocked) Excellent
Original Apple iPhone 6s 2GB RAM 16GB 64GB 128GB ROM 4.7" iOS Dual Core 12.0MP Camera fingerprint Unlocked 4G LTE Mobile Phone|Cellphones| - AliExpress
Best Buy: Apple Pre-Owned iPhone 6s 4G LTE with 64GB Cell Phone (Unlocked) Gold 6S 64GB GOLD-RB
iPhone 6S 64GB Space Gray - Refurbished | Allo Allo (Dominica)
Apple Pre-Owned iPhone 6s 4G LTE with 64GB Cell Phone (Unlocked) Rose Gold 6S 64GB ROSE GOLD-RB - Best Buy
16GB, 64GB or 128GB — Which iPhone 6s or iPhone 6s Plus storage capacity should you buy?
Apple iPhone 6s 64GB price in Iran
Apple iPhone 6s (64GB): Latest Price, Full Specification and Features | Apple iPhone 6s (64GB) Smartphone Comparison, Review and Rating - Tech2 Gadgets
Apple iPhone 6s 64GB Unlocked Rose Gold (Used)
Amazon.com: Apple iPhone 6S, 64GB, Space Gray - For AT&T / T-Mobile (Renewed) : Cell Phones & Accessories