There is no way I'd consider that car without a warranty.

In writing, from BMW. Not "the salesman said"...

You can get all the history in the worlds... the question really isnt WHY the engine was replaced- it is 'what it the risk I get screwed with a brand new, unproven new motor'.

Sounds like you are going to assume that absence of bad news = good news. Massive mistake, IMO

Upper 20s, maybe worth the risk... but only if you can afford it, if you guessed wrong.