Which modules should I buy if.....

[Aegis]

If I want to read up on the rules of 5TH Edition? Short/Long rest, passive perception, etc. Thank you!

[Zacchaeus]

That would be the Players Handbook - it's called the Complete Core Class pack in the store, or on Steam. You can also find (most) of that information in the SRD which you will already have if you own Fantasy Grounds.

[Trenloe]

Also the Basic D&D rules. Included as a module with any Fantasy Grounds licence and also as a free PDF download from Wizards: https://dnd.wizards.com/articles/features/basicrules

[Griogre]

For pure rules, the Basic Rules Module has all of the D&D rules. What it doesn't have is all the character race, class, and PC customization info - but it has all the actual rules. You can also download the PDFs from here: https://dnd.wizards.com/articles/features/basicrules

The advantage of the PDFs is you can search them for Passive Perception, etc.