I think using "dirigeant" for "leader" is not the most approriate.
"Dirigeant" is closer to "manager" while "leader" should be translated as "meneur".
"Dirigeant" is someone who manages, who directs people as he has some authority over them and it is very often used for someone higher in the hierarchy of an organization.
"Meneur" would refer to someone at the same hierarchical level but who naturally guides the people by whom he has been implicitly appointed as the person to follow.
We can use the examples of a music band's leader who is one of the band member while the band's manager is not.
So to translate "He has qualities of a leader", I suggest:
"Il a les qualités d'un meneur"
"Il a des qualités de meneur"
The first suggestion implies he has all the qualities required to be a leader while the second one means he has some of the qualities but not all of them.
Concerning the difference between "les [...] d'un" and "des [...] de" it is because when you mention "les" it refers to the quality of a particular leader so "un" whereas when you say "des" it does not.