The letter “x” was pronounced /sh/ in Old Spanish (spoken in the High and Late Middle Ages) and Early Modern Spanish, covering whatever this website considers “medieval” with room to spare. It shifted to a voiceless velar fricative at some point after the publication of Don Quixote. I’m not sure how the rest of the name sounded in Old Spanish. I get the impression that the stress would’ve been on the 2nd syllable as it is today.