The iPhone 6+ will do fine stand alone and an external GPS is not mandatory, but advisable - see below.
The iPhone 6/6+ have a real nice screen, much better then most boxes.
Internal GPS ist fine and reliable.
BUT, there is one special drawback and that is mobile network. If you have an a/c sensible to electronic distortion, you want mobile networking off. Designwise cutting off mobile net on the iPhone also cuts off GPS, as they sit on the same chip. In this case external GPS is your only way. Given the options I would rather go for a cellular iPad, which does not have this restriction. It is the far more flexible device.
As a backup the iPhone is fine. I usually use an iPad mini as prime GPS and keep an iPhone 5s as backup - both internal GPS and doing fine. The iPad has current SkyDemon on it and the iPhone SkyDemon and AirNavigation Pro as backup, just in case SD has a principal problem. I am flying iOS since iPad2 came out and would no longer consider any aviation boxes an option. IFR certified GPS if needed and tablet/smartphone are the modern options.