Mount's hp bonus is added to each hero individually. Extra hp could definitely help as it increases the chances that Khan survives (might buy you an extra turn or two for him to dodge/block/crit). The stats on your top 2 sets will matter more though.

Mount are limited, so it might be a good idea to get chimera before they run out. Also, if you plan on getting chimera eventually, you should get it as early as possible.