Fonts are really a mysterious topics.

Fonts have always been a complicated subject indeed, but there's no reason why they should be a mystery.

this is really a horror film for a newbie: http://wiki.contextgarden.net/Mark_IV

What exactly do you find horrible there? It's of course not a beginner's guide to Mark IV but I think it shows you some new features, nothing more. There's basically nothing on that page.

- should I know something about the metrics and other trickery?

I would say yes. It always pays off anyway. But it depends on what you want to do. At the very least you should know what tfm files are and where they're installed in your TeX distribution. At most ... well, there's virtually no upper bound on that ;-)

- how do I know which fonts are installed by the distro? AFAIK, it seems that this is a fairly complicated topic.

For ConTeXt, look at the typescript definitions in type-***.tex. These give you high-level access to fonts. Not all are freely available; for example, the typescripts defined in type-buy.tex need font files that, as the name says, you should buy. But most of them are usable out of the box.

Experimenting with

[snip]

Seems to work but I don't know why and how...

These all are defined in the default typescripts. If you only wish to change the default fonts, this is all you need. Otherwise you have to write the equivalent TeX code for your own fonts (not too difficult), and install them in your distribution (this can be quite painful).

- so, excluding "secret" pre-installed fonts, what I have to do to choose a (free) font ? - and how do I install it on my machine? - I.e.: what have I to do? Stay on mark II or switch to mark IV? This "mark" issue has confused me a lot more...:)

Judging by your questions and what you say you intend to do, I don't think you would really benefit from Mark IV. Staying with Mark II is fine, but actually, if your main concern is about fonts, I suggest you have a look at XeTeX; look at the wiki for instructions and examples. ConTeXt with XeTeX is known as Mark III, although the name is much less used.

I think a real tutorial on the font subject is lacking. All the available infos () in some way require IMHO a relevant knowledge

True. But you do realize that your last sentence is true of any TeX-related topic, right? So you should not be discouraged in advance. And what about http://www.pragma-ade.com/general/manuals/mfonts.pdf for a start? If you have access to the TUGboat, the last issue has a great article by Idris that explains the whole process of installing and configuring new fonts. I can answer more of your questions if you want, but you really need to figure out what you want to do. My guess is that you only want to find a set of matching fonts and define a typescript. Try to experiment with more pre-defined typescripts like you did above, and look at the code defining them (it may need some work to find and understand it, but it's worth the trouble); then you can start defining your own typescript and installing your own fonts. Much of this is extremely simplified in XeTeX (both TeX macros and font installation); that's why I recommend you have a look at it too (there, typescript aren't needed at all actually; you can access bodyfonts directly). Arthur