Zetzel's edition of the Cicero's Republic and Laws contains all of the nice extras that go along with the Cambridge Texts in the History of Political Thought. It has helpful footnotes, biographical glossary, and an introduction into some of the difficulties that Zetzel faced as a translator. Zetzel has also supplemented the manuscript text of the Republic with fragments preserved in other writers to give some sense of what is missing.
The translation is one of the better ones I've seen, and I'm not sure that there is a better one. Zetzel found it impossible to translate res publica consistently, but does point out wherever it appears in the text in a footnote. This translation is not suited to the demands of in-depth interpretation, however. Zetzel translates both prudens and sapientia as wise or wisdom, for example, but not consistently. Both scientia and philosophia are translated as philosophy. These problems may cause some difficulty if the text is read too closely, but the general sense of Cicero's works won't be lost.