How concentrated does the analyte solution need to be in order to perform a (chelatometric) titration
Sorry to tell you a bad news. Classical methods, like titrimetry or gravimetry do not work very well at low concentrations. Failure would be a strong word. The beauty of EDTA is that it forms 1:1 complex with most metals. Pb is not an exception. If your lead solution is 0.000001 M, your EDTA solution should be 0.000001 M. Such concentrations are never recommended in classical methods.

For ppm level, spectrophotometric methods can work quite well. Go to Google Scholar and search the methods in Google Scholar