Oldest Iphone you'd still consider really usable and worht buying

I'd go with a 6. He'll be able to use it longer. Plus the screen is a bigger for viewing.

those should be fine..

5c or 5s

My parents each have the 4s and it works fine for them. Granted they don't do anything but talk, text, and web surf.

For people that don't need the latest features, you can get about 3 to maybe 4 years out of a phone before it becomes too outdated. Let's call it 3.5

$650 phone / 3.5 years = $185 per year

If you buy an iPhone 6 now, you get 2.5 years out of it or $550 / 2.5 = $220 per year, so from this perspective you're better off paying just a $100 "premium" for the 6s which gives you an additional year of use. a 5S would have 1.5 years of usefulness, so unless you can get it for $280 or less, you're better off buying the new one.

Lots of people got sticker shock when the subsidized plans started going away, but for the most part you're going to pay about $200 per year to own a "flagship" phone. If your dad doesn't want to deal with that then he should look at Android where lower priced doesn't necessarily mean closer to obsolescence.

How's your battery life? Degrading battery performance on the 4s was a big factor leading me to upgrade last year and my wife to upgrade a few weeks ago.

The batteries on the 4/4S are actually easy to replace. I replaced mine at around the 2-year mark.

Same here

My aunt and uncle are in their mid-70s and had iPhone 4's. They recently switched to Android and switched off of Verizon. They got brand-new phones (Zenfone 2's, I believe) for around $200 apiece,and really, really like the new phones. They are having fun with all of the new functionality. I was surprised that my cousin got them to switch, as I figured they were set in their ways. I think the cost of the new iPhones convinced them to switch, and I think they really struggled reading the smaller iPhone screen.

5S is your answer

Fairly quick, decent camera and TouchID