Consider this for a display:
http://www.tftcentral.co.uk/reviews/aoc_i2757fm.htm
27", IPS, 3-year warranty, $269 @ Best Buy.
IMPORTANT: this is a "medium resolution" 27" display. That is to say, it has a larger pixel size (.311, I believe) -- it shows you the same image as a 24" display, but "blown up" a little bit. What this means for your mom is that she won't have to squint to focus on display, if (like me) her vision is not the best any more.
Combine this with the Mac Mini of your choice, and she should be good to go, at a decent price.