Hmm, that's always a tough question to answer. If we talking casual home use for internet browsing, E-mail and maybe watching some movies. I would say get the base configuration iMac for $1,099 but upgrade the storage to the 256GB Flash model. While the 5,400rpm Hard Drive will get the job done. I think the upgrade to flash storage will allow you to keep the iMac running longer. Especially if someone is keeping it for 6+ years as a long term investment.
Even the Ultra HD 4K resolution, $1,499 version comes with a 5,400 rpm drive which i feel is wrong. At least put a 7,200rpm in there. Not good value there.