To start, I would choose systems and languages that have, so far, stood the test of time as there's no reason to think those won't be around for 100 more years.

To start, I would choose systems and languages that have, so far, stood the test of time as there's no reason to think those won't be around for 100 more years. I would write the code to solve the exact problem and not try and create some application framework that would allow for any feature request because you don't know what is going to happen in 2 years much less 98. I would do the absolute simplest thing necessary to solve the problem. The goal is to minimize the surface for bugs to manifest. I would use plain text for storage. I would strictly avoid anything "flavor of the week."