Do I have to add a second module SSD into this laptop (LG Gram 17) before installing Manjaro?
You can install Manjaro alongside windows if you want to and dual boot from the same SSD. I would prefer to install it on a separate SSD cause windows usually doesn’t play nice with Grub.
I rather want to replace Windows with Manjaro.
Excellent choice! No need to add another SSD then. Just follow the calamares install instructions.
Ensure everything is working using a live ISO before wiping Windows.