I'm looking for manga where there's a fundamental Buddhist concept through the entire series. That may or may not have monks and nun and secondary or main characters and may or may not have reoccurring reincarnations.

I'm not looking for the entire autobiography of Buddha
what i'm looking for is something like Inuyasha where there is consistant buddhist theme throughout and Kyoukai no Rinne where the story takes place based of buddhist motif.

keep in mine that i'm not looking for shinto theme (ghosts and oni and so forth), although due to the nature of Buddhism in Japaneses history, the two are rather hard to distinguish (take Shaman King for example

hmm, would you be also interested in Hinduism? (both are often practiced together)

3x3 eyes has some references to a handful of Hindu gods, although most of them occur as monsters/gods of destruction. I think I just liked it a lot cause I'm largely unfamiliar with Hindu stories and having it set as the backdrop for this manga was pretty interesting for me.

Genjuu no Seiza?

Totsugami

________________

Amaenaideyo!!, and it's sequel Amaenaideyo!! MS

Nijigahara Holograph
Probably offtopic as well, as it's loosely referring to Taoism and is set in modern Japan (no epic/historical background). But it's full of symbollism and sure takes one's mind off for a ride if it's also what you're after. See OppKnox's review "On Butterflies and Taoism".