For just surfing and e-mailing I recommend my customers to get the cheapest one they can find. ATM the cheapest desktops here are branded by Acer and Compaq, have 4 gigs of RAM and some 500'ish hard disk, cost about €350 including 23% VAT. Also, our consumer protecting laws extend the manufacturer's liability to 2 years, so that would be the minimal time they'd last. Given that elderly people might not use their computers every day, their lifespan could be well over five years which is an eternity for a computer. I can't get the components for those prices, not to mention the OEM Windows which costs about 20 instead of 99 for the big manufacturers.
One considerable thing is, that with a ready built one you don't have to worry about the quality and warranty of each single component. And, if you're running a business, you don't have to check for free whether the rig should be sent to warranty repair or not.