Mine was a baseball game on an Atari 2600. Couldn't tell you more than that, my granpa was super into baseball and tech so he bought an Atari brand new. We dug it out years later and played.
Not a GD, so here's my 2 cents on the top 2 - build from scratch. It will be cheaper in the long run as you can have an upgrade path. Some SIs use proprietary parts that will screw you later.
Any non-rolling distro will be good. If she is a long time Windows user, I would recommend making it have KDE for the desktop as it seems to be pretty Windows friendly in orientation.
Mine was a baseball game on an Atari 2600. Couldn't tell you more than that, my granpa was super into baseball and tech so he bought an Atari brand new. We dug it out years later and played.