Each season is completely self-contained. There is no separate "base set" which needs to be purchased. If you buy the 1985 season, for example, that purchase will include a download for PDF files of every team from the 1985 season, all the charts specific to the 1985 season, and all of the game rules and charts. If you then purchased the 2022 season, that download would also include all of the game rules and charts.
The game is currently available in PDF only. If the game continues to grow in popularity, I will look into making physical copies available. Currently, trying to sell limited copies of physical cards has been cost-prohibitive and I've instead chosen to make the PDF files available for cheap and leave it to consumers to print out cards on their own if they'd like.