Hi
After some research I found this:
https://openwrt.org/toh/hwdata/friendlyarm/friendlyarm_nanopi_m1_plus
I think can be a good compromise for me (price, specs, etc)
Is also a good hardware for installing OpenWRT?
In the Installation method(s) of the wiki I cant find other information, can I use the integrated 8GB eMMC for installing OpenWRT?
Thank you!
What exactly are those?
Looking at the images provided for this device, SD card is the way to install.
https://openwrt.org/docs/guide-user/installation/installation_methods/sd_card
As frollic mentioned, it highly depends on the use case.
For IoT maybe.
But then you would be better off with some generic Linux distro like Debian/Armbian etc.
With only one LAN port and Wireless 802.11 b/g/n it hardly can be a good router IMO.
My needs
I need a small device, no more expensive that I can install a working openwrt with no problem (software or hardware for performance)
you're too vague.
define
plus performance.
based on your (non existing) requirements, get a cheap MR70X, AX23, WSM20, DAP-X1860, RT1800, etc ...
Great, let me explain my needs:
I'm looking for an OpenWrt router that can:
I hope this clarifies my requirements.
still need to know the speed of your internet connection 
and since the wireless appears to be coming from Broadcom, expect it to be really crappy.
I have a 100Mb connection.
The range can be not a real problem, because I need this WiFi in only 2 adjacent chambers and in one of these I will place the OpenWRT device.
Do you think the GL.iNet AR300M16 would be a good fit for my needs?
Even without VPN, this AR300M16 already can't fit the bill, this router is simply very slow, and with your budget I cannot find anything that can fulfill your requirement as well.
assuming WG is supposed to run at 100mbit.
Ok
What device you suggest me to use?
at €30, with your expectations, nothing, as @fakemanhk already pointed out.
if you're OK with something used, the https://openwrt.org/toh/google/wifi#installation could perhaps fit the bill, they're cheap on eBay.
look at A Wireguard comparison DB, to get an idea about what kind of SoC you need, to be able to run a WG tunnel at 100mbit, keep in mind routing requires CPU cycles too.