Greg, in addition to the commentaries already cited by others above, you might want to take a look at J Patout Burn's commentary on Romans
in The Church's Bible collection. I have not examined the volume, but the purpose of the series is to introduce readers to the exegesis of the Fathers.
Among contemporary commentaries, take a look at N. T. Wright's "Paul for Everyone" commentary on Romans
. Wright also has a longer commentary on Romans in the New Interpreters Bible commentary series.
You mention wanting something specific to read on the Apostle Paul and justification. Take a look at my recent blog series on this subject, beginning with "Eastern Orthodoxy and the Apostle Paul
." Unfortunately, Orthodox theologians and scholars have not written very much about justification, preferring instead to skip right to theosis. This, I think, is unfortunate--especially given the fact that justification is an important theme for the Apostle. (Hint: when reading the Epistle to the Romans, read chapters 1-4 through the lens of chapters 5-8.)