As to who to play Siddhartha - the Prince who becomes the Buddha; this brings in two competing views: One would be to go with a top young A-list actor, preferably but not necessarily a Buddhist. A top young Buddhist actor could have an added personal boost to the energy and care needed for the role. If going this route, then possible other Buddhist actors would match up.
The second view is to go with a relative unknown, as they did with Gandhi and Ben Kingsley. As Gandhi won eight Oscars, with Kingsley winning his own, the quality of The Great Quest would provide the opportunity for a talented upcoming actor to shoot to stardom.
The double lead role of Siddhartha/Mara is very powerful and, no matter who does it, if done well, is Oscar material. Once the script goes out, there will be many actors wishing to do it.