As far as the language of historical texts goes, the emphasis is on the language of inscriptions in the SEAclassics project (often difficult to figure out, requireing specialised dictionaries):
"Southeast Asia's golden age of epigraphy spans more than a millennium, from the 5th through the 15th centuries. The SEAclassics Library of epigraphic texts, Indic and epigraphic dictionaries, and research-oriented software tools will make this widely scattered body of work, including the Cham, Mon, Khmer, Pyu, Burmese, and Tai inscriptional corpora, accessible to the international scholarly community. A demonstration of the Corpus of Khmer Inscriptions is available on line."
Don't forget to read the intro for students before you begin