for cable lengths less than 15 metres, all HDMI cables are equally capable for carrying video and audio signals without issue. since signals are digital in HDMI, you either receive the signal or not. "better picture" is subjective and would only pertain to analogue signals like those carried across composite or component cables.
the upgrade is not worthwhile given your primarily usage of the iMac.
assuming we're talking about the desktop i5, single-threaded apps will only run on a single active core (the other three cores will be down-clocked close to 0GHz). even though the remaining core will be overclocked to 3.2GHz maximum that won't help you read email faster or download content from the Internet any faster / better than a top-end i3.
i suspect, for senior citizens (aged 65 and above) especially, their reduced dexterity makes touch screen interfaces a bit tricky to use. also, font size and clarity becomes a much more important consideration as one grows older.