my two cents:
I am also 5'7" and ride a 54cm road bike. My commuter is a 19" and I did at one point have a 19" mtn bike. However, my DH thought it was big on me and I did hate that bike (which may or may not have been related to its size).
Have you test ridden any bikes with similar geometry? Why this bike and not any of the other thousands on ebay? My recommendation would be to look around locally for second hand bikes or maybe even new ones on clearance and get a good idea of what you want. If you can't find it near you, then check ebay. There doesn't seem to be anything special about this bike and there are lots of upright commuter options. I would wait until you have a better sense of what you want before making a purchase, but that's just me.
...never met a bike that I didn't wanna ride.