Some question about flashing a ROM for my Samsung (newb here)

So I have the Galaxy S5 SM G900F, running Android 5.0 , no more software update for this device, well at least in my country.
It has become very laggy, many apps crashes such as Spotify and Snapshat and it just sucks overall. I'm planning on installing the 6.0 marshmallow ROM following the steps from this thread hoping it'll fix the problem.

1- Does it matter which country I should download the firmware on sammobile? Because in Lebanon, there's only the 4.4.2 version, even thought I have the 5.0

2-In the thread, the OP said " flash stock Android 5.0 Lollipop firmware first before upgrading to Android 6.x.x Marshmallow firmware to avoid any issues." What does he mean? To do a backup of my existing firmware? If so how should I do that?

3- Would my data such as pictures, contacts, apps be affected?

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