Off the shelf, out of a drawer or box, that sort of thing... I am a halfway decent "gun plumber", which is pretty much all it takes to put together an AR.
Of the two I am familiar with (G17 and G19), I prefer the G19, so I would think that would be a good start. Although there is merit in Bladite's love of the Springfield XD(m).
I'm still predicting that 40 S&W is a fad... I'm sure you've heard it called 40 "short and wimpy"... I've always thought it was a compromise that wasn't executed particularly well. Although my brother-in-law, a County Sheriff, loves his G22 and says the G35 makes a good shooter into a great shooter.
I've also heard that the G30sf was the G30 "short fingers"...