As Cvolton says above, lighting is important.
You'll get OK footage from an iPhone 6 if you have decent lighting. If you cannot afford to buy lighting at the moment, record during the daytime so that you take advantage of natrual light.
Be aware, however, that clouds and bad weather can ruin your video during the day; especially if you stay in a country which doesn't get a lot of sunshine (I live in Scotland so I know what I'm talking about !).
Another issue is audio quality. I highly recommend recording audio separately using a good mic. For example, a lav microphone, an audio recorder, or a dynamic mic like the Audio Technica ATR2100 (that offers good quality at an extremely low price).
Make use of a tripod where possible too.
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