Gday @SignorCastello , Try searching the forum as there are several topic's.
Try this to start with.
Look here for system requirements,
Best advice is " Try " core , then " Try " Lite,
prior to a full install, Note that thing's may run a little slower via usb drive , but runs quiet adequate for testing & get the feel of the OS.
@storm has been making theme's lately & would be a good one to talk to. There are several other's that have diff theme's/desktop enviroments, they maybe offline atm.
i think i covered all 4 question's, some answers are within the link's.
eg: do i install first & thing's to do.... there is alot to learn & master what needs to be done
It would have been useful to have full details of your "low end spec" machine to give a sufficient answer. As an example my good lady's machine, a rig I put together in 2006, is maxed out at 2 Gb RAM with a single core AMD Athlon64 processor and EVGA silent cooled AGP 8x 256 Mb nVidia card so can only run xfce version of MX-Linux and not the KDE version.
I'm sure I can run kde, rn I'm on manjaro kde, but I cannot use anything but zorin. Should I download the core or the lite? then I'll install kde and use only that.
The only difference you would see would be the amount of space taken up on the drive.
Since you are planning to install a D.E., it is that D.E. that would determine the performance of the machine.
Core is a bit larger than Lite; so you may opt for Zorin OS Lite just to have a smaller base footprint.
Yes, this is how I would do it if it was me. It would be safer and simpler than modifying the .iso prior to installing Zorin OS.
I cannot think of anything particular that stands out as a warning. Installing Plasma D.E. is pretty straightforward.
Plasma runs more lightly than Gnome. This is likely most optimized in the latest Plasma version.
This second guide working 100% i tested.
When you doing a compile kernel it is good idea to enabled zcat.
That help you more configuration kernel update or patch from a book Robert Love Kernel Linux Distribution and linux kernel i nutshell.
The instruction
Download Zorin Core.
Installing on hard disk. (1 host)
Create a VM install Zorin Core on Vm (2 host)
Install a ssh on virtual machine.
Then from instruction from links what i gived you start creating distribution and compile kernel.
Here is guide how update a kernel after compile. How To Build Linux Kernel {Step-By-Step} | phoenixNAP KB - not tested
I found also on linux mint guide how from virtual machine what you created you can build your own image.iso with choosing which package you want install from example Zorin. Mint and Zorin are ubuntu then guide is the same the diffrents are desktop enviroment.
The best option to compile a kernel are processor Intel Xeon E7 or Intel Xeon E5-2697 V2 or Xeon E5-2695 V2 and some good size a ram.
The people talking best tool for creating distribution is that tool. Remastersys download | SourceForge.net