As per title I'd like -starting from ubuntu command line system- to setup a system with bare minimum kde installation.

I don't need the "semantic search " function, but I'd like to keep the "simple seach" (simple indexing, and finding files given their name).
To such purpose I'd like to know which component is needed for the simple function (for instance, is virtuoso needed? Since it's possible to not install it..), and possibly which services too (blocking them from the configuration files, like Akonadi and Nepomuk, which otherwise I don't think I'd need them running since I don't use any of the kde PIM application)

Ok, after some trials it seems that Virtuoso packages need to be installed.
I have setup 2 vm, starting from ubuntu server command line installation, and in the one one which I installed kde-plasma-desktop excluding the recommended packages (among which it appears Virtuoso-packages) the indexing search is broken.

Unless someone can prove this wrong.

I can confirm that Virtuoso is the currently recommended backend for Desktop Search - and as such is required.