Thank you for the comment, tropicalfridge.
The question evolved from an opportunity to become instructor at the flight school I am currently in. I hold PPL, SEP rating and 50 flight hours. Have just passed ATPL ground studies and the 14 exams. Will start my flight practice with a DA-42 soon - in the mean time there appeared this whole thing with the PPLs with FI ratings - which is a good point for me for hours building (not to be paid) - not as a long term career so far, although if after getting my CPL(a)/ME/IR there will still be a chance to upgrade my instructor rating to ME/IR that will be a good start for an aviation career in my eyes.
The question still is - how many hours do I need to be eligible for the FI rating?