Maho MH 300 C Retrofit...?

If I wil going to use a desktop (for Linux Software). 7i84 is for what??

A 7i84 is a Mesa Smart Serial card and adds additional isolated digital IO points (16 out and 32 in), essentially doubling the the IO available with only the 7i77. It may or may not be necessary with your conversion.

Good Morning!
I tried to install linux CNC on an old pc and an empty disk but the best I could do was to have normal linux operating system installed on my computer. I thought I was going to stick with something similar to this I send in the photo.

If you installed the linuxcnc iso, then you have linux + linuxcnc. In the menu you do not have a group of programs that are named linuxcnc ?

In the picture that you sent, it is linuxcnc running gmoccapy, that is a "front screen" to linuxcnc that can be customised.

Yes, I already knew that. Subsequently in what way is it possible to put LinuxCNC (after customizing) to communicate with 5i25?

Not sure I understand your question, but Linuxcnc communicates with Mesa cards (such as your 5i25) via the Hostmot2 driver software.(which is included with Linuxcnc.)

(You can build a Linuxcnc config for using Mesa cards using the PNCconfig wizard.)