well, if this is any indication of what BTVS is like, i think i'll pass (god, is she always this annoyingly melodramatic?!?) ...
Buffy comes off as far more of a bitch in her Angel
appearances than on her own show. There are good in-story reasons for this, but it's unfortunate from a cross-promotion perspective.
although... is there a Buffy ep that serves as the backstory to Five by Five and Sanctuary?
Yes. The episode is called "Season 3". (The two-parter "This Year's Girl"/"Who Are You?" is the immediate setup; actually, those two together with "Five by Five"/"Sanctuary" are sort of a 4-parter, but even so you need a lot more of the Faith arc to really appreciate everything.)