I wouldn't want Banda over Peralta because Banda doesn't have options and Peralta does. Rob Z and Banda are a toss up. I'll keep Rob Z as he's been quite good as a Brewer. Ashby is a leverage guy when in the pen. Not sure where you're getting the idea Koenig isn't good down the stretch from. Here are his numbers as a Brewer.
August - 20.1 IP, 2.66 ERA, 2.31 FIP, 3.10 xFIP
September - 22 IP, 1.64 ERA, 2.79 FIP, 3.72 xFIP
Overall numbers as a Brewer - 128 IP, 2.67 ERA, 3.31 FIP, 3.87 xFIP
So Koenig has actually been better at the end of the season than he has overall as a Brewer.