Adding boolean variables for each item only scales so far, your inventory class has to know about all the items in the game.
The next simplist implementation would be to change the inventory to a list of strings, then your level can check if the list contains a given string without the inventory code needing modification.
If you need to track item counts (e.g. the player has 5 apples), then you can use a dictionary instead of a list. Then you can use the string as a key and store an integer as the value.
And presumably the researchers count as two of these 15 downloads.
Its not great that they were up for so long but luckily these were no designed to attract users.