Apples aren't that expensive for what you get, especially refurbished. Plus a mac mini is way cheaper than that thing he linked.
For updates, can she handle clicking a single button? On os x when theres an update available it pops up and you just click the install button. No control panel to go through, etc. Also you mention her needing a trackpad, and the magic touchpad is pretty much the best you can get. I have arthritis in my hands and can't handle using a mouse, but the magic trackpad is nice and big, so I'm able to use it without issue.
Depending on the resolution of the display you connect, reading can be hard. I have a 23 inch monitor but at its native resolution its hard for me to read without my glasses even close. Really the best thing for you though is to have her try it herself and decide. You can go into an apple store and try an imac with the magic trackpad and see if it works for her.
Heres the accessibility features in case any seem like they'd be useful to her: »www.apple.com/macosx/what-is/acc···ity.html
As for making it simple for her, I don't think you need simple finder. Just put the programs she actually would use in the dock so that when exited they stay there. Remove any she wont ever touch. Maybe resize it to be big so its easy to click. Also if she can manage it, I find two finger scrolling on the trackpad to be much more comfortable than clicking an arrow.
As with anything the only way you'll find out what works is to try it out. No matter which you go with, I'd consider remote software such as logmein so if she needs help you can do it remotely.