Applying to informal call for PhD students
I suggest you contact them with a letter of interest as soon as you can. You should have a CV and a SoP in good shape, but you probably have a short time to refine it if they show interest. Letters and such can probably be safely delayed for a bit until everyone is serious. 

But, since they ask for references, names will probably do for the moment. But you should contact those people so they know that a formal request is forthcoming. 

But, if this is an informal sort of thing, rather than a formal application, you can probably ramp up the details as they emerge. 

It is a good idea, of course, to keep your CV up to date at all times. It is painful to have to put one together quickly and you are likely to forget some important things. 

I'm assuming this is pretty much a universal thing. Not limited to EU. In some places, I think including the EU, applications often start and sometimes end with a PI. So, it isn't wise to be too informal.