Short answers, pulled from the story.
Dio spent ten years collecting material on Roman history up to the death of Septimius Severus in 211 AD, then twelve more years composing the work, for a total of twenty-two years on the project.
Dio wrote in Koine Greek, even though he was a Roman citizen who held senior Roman offices including two consulships.
Books 36 through 54 and Books 56 through 60 survive nearly or fully complete. The first 21 books have been partially reconstructed from fragments and the epitome of Joannes Zonaras. Books 22 through 35 were already lost by the medieval period. The later books (61-80) survive mainly through an abridgment by the Byzantine monk John Xiphilinus.
Dio served as a senator under the emperor Commodus, governor of Smyrna, suffect consul around 205 AD, proconsul in Africa, proconsul in Pannonia, and consul a second time in 229 AD under Severus Alexander.
Dio's Roman History is one of only three surviving written Roman sources that document the British revolt of 60-61 AD led by Boudica, making it an essential primary source for that event.
Several Byzantine scholars preserved Dio's work: Joannes Zonaras used it as a primary source in the 12th century, John Xiphilinus abridged Books 35-80 in the 11th century at the request of Emperor Michael VII Doukas, and fragments were gathered by Henri Valois, Nicolas-Claude Fabri de Peiresc, Fulvio Orsini, and Angelo Mai over subsequent centuries.