Hi everyone, so I was wondering if I for example buy the game's product key from an external source (instant gaming, Cdkeys,G2A) will I still get the wallpapers and other stuff included with the game?

I do not recommend this. I can not tell you if you get bonus stuff when you purchase the game on non official "stores" like this. I highly recommend to purchase the game here on GOG or the game in the box (GOG code is inside)
to support CD Project Red.

I want to support CDPR to the fullest so I think I'll buy it later on as I wanted to save those 20 euros for other games. It's just that I think 60€ is a bit much for a pre-order.

Look at physical retailers in your country. I've already seen one that offers it for €55 here with free shipping. The physical copy is essentially a GoG key in a dvd box.

G2A are very dodgy, avoid them. I have bought codes from CDKeys and never had any issues.

A lot of keys on those sites are bought with stolen credit cards. Credit card companies will charge that money back from the game developers and they will charge them transaction fees. When you download an illegal copy of a game, that doesn't cost the game developers anything. When you use a key from these sites, it often costs money to the game developers.