Can you people help me make a centaur shaman combat medic?
Sure. Charisma is the only attribute you need to specially care about other than the normal ones (Willpower, Intuition, Reaction, Agility, in about that order).
I want to make a magical healer with combat spells that does summoning and talismongering. We are using a sum to 12 system with prime runner rules.
Get Summoning, Spellcasting, Binding, Counterspelling at a decent rating. Ditch talismongering. The rules are best for NPCs. If you want to flavor it up, use fetishes for your spells.
I want to use a Ares Archon Heavy MP Laser or a Ares Thunderstruck Gauss Rifle, which one should I pick?
Laser, because then you can use the laser shotgun, laser pistol and laser SMG.
Should I be a mage or a mystic adept?
Whichever you want. Mages can astrally project. Mystic Adepts get adept powers. I like mages.
Will this build work?
Yes.
What vehicle do I want that will work in the mountains?
You don't need a vehicle. You're a horse.