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I will start by mentioning what other equipment I have around me at this point:

Soundcraft Sound Signature 10
Faderfox Pc & Mx 12
Iconnectivity Mi010
Circuit Happy Missing Link

These are units that, more or less, I think they could take part or not in the future discussions.
-- Tnsl

I'm not sure what you mean by that. The Soundcraft is a mixer so no issue. Faderfox, Iconnectivity and Circuit Happy are all MIDI devices though, so it really depends on whether or not your Eurorack setup can accommodate that much MIDI input and if you want to spend HP for modules making the MIDI to CV conversion.

My first question is: How much importance should I give to this specific case knowing that one row is 1u and It Has to be intellijel utility modules? Should I go on a case without 1u modules ? How efficient are those Intellijel 1u modules to cover the needs of a beginner ? Is it safe to invest in a case like that, knowing that I would have to use Only Intellijel modules for that particular row?
-- Tnsl

The way I see it, the 7U isn't such a big case (but that's highly dependent on the build I personally have in mind for myself, a very personal perception of what is needed in a rack). Therefore, any HP I can save in the 3U space is welcome, and the compatible (again, watch out for Pulp Logic vs Intellijel) 1U offering by Intellijel and a couple of other manufacturers helps me tremendously. I'm particularly fan of the O&C in 1U format, as well as the quad attenuverter, couldn't live without those (I have 2 now, thinking about getting a 3rd). Something to consider: cases with a 1U row seem considerably more expensive than their equivalent without, but 1. you save space and 2. sometimes the 1U module is a bit less expensive than its 3U equivalent.
As Lugia said, Intellijel is not the only player in the 1U Intellijel space. The offering is limited but there is already quite a lot of interesting utilities in my opinion. What is less "safe" is the "lock in" situation: once you get into 1U, you will need to get rid of all your 1U modules if you pick a case without that row, or you will need to limit yourself to options with a 1U row as well (I went with that when I evolved out of my 7U).

If you are worried about the usability, I find that the 1U row on the 7U is ideally placed and the modules design is usually taking the form factor into account so that's not much of an issue. But if indeed you can't see the usefulness of an attenuverter, a linear VCA or a slew limiter, please do follow Lugia's advice and get some experience with free software, it will help you make a better plan based on YOUR patching techniques and related needs.

Hopefully this helps. Welcome to MG :)