Usually according to the "Charlton Standard Catalogue" Low Serial Numbers (Categorized as N5's) are valuable from 1-999. BUT its the lower the better.
As for highs there isn't anything really special or sought after, UNLESS its just happens to be the very last note of the series.
N5-i represents #1
N5-ii represents #2 to 9
N5-iii represents #10 to 99
N5-iv represents #100 to 999