I would recommend the iMac for simplicities sake. Less cables, comes with KB/Mouse too.
My mother got her iMac in August 2007, and still uses it to this day. Only issues she's had has been with lighting strikes going through the internet line before we used wireless back in '07-'09. Both PSU and GPU were replaced under Apple Care though.
It was the base 2.0GHz model that shipped with Tiger
Upgrades over the last 8 years would be 2GB to 4GB RAM, and a 128GB SSD over the stock, old, slow HDD. It's been through every OS X release from 10.4 to 10.10. Never a hiccup.
She isn't a power user, but she uses Mail, Safari/Chrome, iTunes for her music and iPhone, Office 2011 and Pages '09/newest one, and some iPhoto.
I'd just pick the middle iMac as it is, perhaps with the 256GB SSD for the future and to reduce a point of failure. Should be good for the next decade. I think my mothers iMac can make it through two more years easily.
Ain't she a beauty, for being an 8 year old computer
As for the Mac Mini:
It is cheaper for the unit itself, but you have to factor in the cost of a decent monitor, keyboard, mouse, and speakers. Also some cable management would be needed.
On the plus side, they are easy to get into (with the right tools) should something fail or need replacing years down the line when it's out of warranty. Though the 2014's have soldered RAM, the SSD is replaceable-ish as is the power supply.
As I stated with our iMac, it wasn't bought as is and left alone, it was upgraded in 2012 with more RAM because of Lion and ML using so much, and then the SSD in 2013 to help it along with Mavericks and Yosemite. It was the base model however, so for future proofing with either the iMac or Mac Mini, go with 8GB RAM now, since it'll be stuck with it later.