Buffy is an immortal Slayer.

I'm not sure if the original poster is still looking for these stories after five years, but here are a few more where Buffy is immortal in some shape or form.

For Her by JackOfSpikes
The Truth is Out There by Jackofspikes
The Order of the Light by JackOfSpikes
Replacement by Sus
Night Child by Hulettwyo
Consequences by Forever Me
The Pathway To Hell (Is Paved With Good Intentions) by Jamies_lady
Out of the Ashes by Revello_1620

Thanks Much for the update - I plan to read these.