That Claude Butler is a very nice mixte. Buyer beware with that seller, he has enough marginal and negative feedback that I would really want to do a local pickup.
I look for bicycles pretty much every day. I have found five mixtes in the last two years (at reasonable prices). My Centurion is the best one I have found. A couple were high ten steel frames, one Carbolite 103, one generic cromoly, and the Centurion. Mixtes are really hot in my area. So if you see what you are looking for, I would be ready to pay a premium for it over a comparable road bike.
Other than that, what is your budget? My Centurion Lemans mixte is pretty decent (I am keeping it). It has Champion 5 Tange frame, has a frame mount for the RD. It started as a frameset find off Craigs List.
With the help of a donor bike, it ended up like this: