You’ll probably want something as a hub—HomePod mini can work for that—and the rest is up to what you want to spend and on what you want to spend it on.
Lightbulbs are available at various prices including USD$30 or so, with other options depending on what you’re doing. Lighting and sound systems are among the most common HomeKit options, though there are various others including water and temperature and humidity sensors. HomeKit appliances are rare, and I’m not sure what you’d even do with those.
I’d also have to ask whether the folks are going to be able to contend with this too, as it’s all going to be very different from what they’re accustomed to, and here will be some fundamental differences—such as always leaving the power switches on for at least some of the converted devices—that will require some adaption.
HomeKit is fairly rare in major appliances, and major appliances themselves can be somewhat difficult to get right now. A ~six month delivery schedule is not unusual for some common not-HomeKit-enabled home appliances, too.
You know these folks best of course, but I’d be very cautious with this general goal. Not all that many octogenarians are computer savvy, and remote support is going to be frustrating for all involved when somebody can’t turn on a light or a HomePod gets stuck after an update (as some just did) and needs to be un-paired and reset and re-paired.