It is partially closed source, but most of it isn't. The firmware upgrade process has been simplified to the maximum. In the past you needed to connect, disconnect, and reconnect the cable again while holding a combination of buttons. They abandoned that and now you connect the device, click the download button, confirm that a serial code on your device matches with what is displayed on the screen and you are ready to go. Installing/reinstalling apps is a 1-click process, completely beginner-friendly.

Since you intend to use it for Bitcoin and Monero only, the older and cheaper Nano S is good enough. You will have enough memory for a 3rd/4th app even after those 2 are installed. But, if in the future you want to keep 50 different coins on it, you can always uninstall one to install another one in its place. It doesn't affect your holdings in any way. 

You just missed a Ledger Nano S/X promotion where it was possible to get a 20% discount on your products. It expired yesterday.
If you decide to buy one of these, Ledger often gives discounts on their social media. Sometimes it pays to wait a bit.