Do you need special thickness ? (i.e. is thicker better/worse) ... Does the proximity to the substrate matter ? Does the dielectric constant of the surrounding matter ?

No special needs for the thickness (1st approach). No issue with proximity to the substrate

I'm not sure what size of spacing / features you need, but I think you could pretty much go with any of the metal layers between 1 and 4. metal 1/2 allow for smaller/finer features. metal 3/4 are a bit coarser. A quick look at the DRC rules I didn't see anything that would prevent you from doing what you want to do.

Ok thanks . I will test to draw a design with this metal layer. Do you know how to make the link beteween the design track in metal layer and the pad (pin ) of final hardware component ?

Well "usually" you have something like a 100ux100u pad on the top metal layer with an opening in the glass/passivation layer that you bond to. But here in this instance there will be a harness provided that will include the IOs but details are yet TBD AFAIK so no info just yet.

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