reducedfatoreo, I'm prejudiced for the 600 group soI say put a 600 RD on it, but there'll be little practical difference. Now that you have the FD working I think you'll find the 52/53 large chainring plenty for your cassette. I run a 52/53T X 13T top on my road bikes and it's plenty for flat land riding. The two extreme gears are the least used followed by their respective runner ups, in other words you'll use the gears in the middle of the cassette most often. Ride for awhile and evaluate.

Brad