I certainly don't think it's inconceivable. Buffy in S1 is hardly an exceptional fighter. She doesn't really take a noticeable step up until she drowns and comes back against The Master. Prior to that she's certainly not "bad", but she's not even remotely close to the fighter she becomes later on in the series. I mean, Luke pummels her in the first episode, and while she defeats him in "The Harvest", it's more due to Luke's remarkable stupidity then to Buffy's fighting capacities. She's totally unprepared for The Master when she arrives in his lair (perfect example of dumb Buffy there).
So yeah, without Angel's intervention I could see Darla being able to kill Buffy at that point in time. Later in the series? I don't think so, but then, yes.