Endless Sky
According to wikipedia it is a space trading and combat simulation game. Its free and open source, has a lot of content (even more with plugins). You do missions to get the storyline forward and to get money, you can also mine asteroid, trade with other planets, attack other ships and plunder them. You discover new species and Outfits to make your space ship better, etc.
Yes, but I use the cloud for other purposes too and have enough storage for a backup, also even if I were to buy another drive, I would like to store one copy offsite without having to buy two drives
Some addition to my post:
The computer I found is a Fujitsu Esprimo Q957. I've read online that they are very power efficient and don't draw as much. I will probably run some lightweight task almost 24/7 and sometimes heavier tasks for short periods of time. A raspberry pi is for me too expensive for the average specs, even if it is very efficient. I think a upgradable pc or mini pc is best, because they are cheap, but can be easily upgraded without buying a whole new computer.