It might be a good idea to plan this out a bit...
Centaur Builder
220 Hp
Builds: Altar, House (tent?), Barracks (Orcish? What model would it use?), Caster Barracks (Again, what building? Another tent?), Town Hall (hmm...), and defensive structure.
At a bare minimum, I think you need those things for it to qualify as a 'race'. You don't necessarily need a siege workshop, or an aviary, or a shop... but those would help, can't really think of any 'flying centaur' units though.
Centaur Tier 1
Strong Melee Unit (like a grunt? 700-900 hp, high gold cost, uses centaur melee model), ability: Cleave? A stun? What would be interesting?
Light Ranged Unit (uses centaur archer model, 400-500 hp, medium gold cost), ability: Searing/Cold Arrows? Or maybe a new arrow type? Poison arrow?
Tier 2
Siege? A ram of some kind? Or maybe a barrens creature?
Caster(1): Support type? Maybe different kind of healing spell, or rejuvenate if you're being boring... purge could also work, again if you don't want to use new spells. I always try to use 1 autocast, 1 situaional cast (like invisibility or unholy frenzy) and 1 'ultimate' cast - something like an aoe though I try to stay away from. It's hard for me to suggest anything without knowing more about the map in question though.
Caster(2): Offensive or defensive type... something like frost armor, cyclone or curse, or something. Of course that sounds boring, but again, it's hard to suggest spells without knowing the make up of the other races. Heck it's difficult to make any race knowing so little about the rest of the map's constraints (how much food does the average unit consume? how much gold/wood? Combat stats?)
Heroes:
This is the trickiest part because by now we've used up all the centaur models in the game.
At best we can make use of
http://www.hiveworkshop.com/forums/models-530/centaurarcher-121926/?prev=search=centaur&d=list&r=20
http://www.hiveworkshop.com/forums/models-530/centaur-103826/?prev=search=centaur&d=list&r=20
Though they're on the big side, and that will still leave us with at least 1 more hero to divise....
Unless someone knows of some centaur models lying around, or unless you don't mind reusing different tinted models, or if you don't mind the race containing harpies, quillboar or other barrens creatures... then you're out of luck I think.
Tier 3
Again, we're out of models to use, and I can't really think of any good tier 3 centaur units, they wouldn't exactly be distinguishable from each other. At best you could make it so when you reach tier 3, the tier 1 units can get 'troll berserker' like upgrades, so you can make tier 1 units more comparable for tier 3.
At the very least, explain your map some more, what are the other races like. Is the map closer to WC3 Melee in terms of damage/health ratios? Are these centaur nice centaur or evil centaur? How developed do you want it (for instance, the naga race by blizzard only goes into tier 2 - they only have 1 caster unit, 1 air unit, and no tier 3 units).