to Windows or vice versa unless he wants to bring that upon himself, especially if he is older than 70.

The transition from Windows to ChromeOS is easier, but could still be frustrating. He won't have access to any of his installed programs anymore and will have to replace them with browser based alternatives.

A Chromebook will work for anything that is web based, but some genealogy people still use locally installed software. He's not going to be happy if he can't use that specialized software anymore.

All three operating systems are capable of driving a large external monitor. If you buy a Windows or Chromebook machine then you'll have to pay attention to what video output ports it has. If it has HDMI or DisplayPort outputs you'll almost certainly be able to drive any monitor you can find. If it only has USB-C outputs then you'll need research carefully. A thunderbolt 3 port will be able to drive any external monitor, but lesser USB-C ports sometimes have limited video support.

A mac-mini should be able to drive at least one external monitor of any type with the right adapter.