When it comes to Age of Magic, I don't think you need a spell caster. I think it should be fairly balanced to just have another point of troops. You will have fewer tactical options.
My preference for proxying Middle Earth forces in SAGA is the Age of Invasions book. Romans and Britons make good Gondor or Arnor proxies, Visigoths could be nice for Dwarfs, I use Ostrogoths for Rohan, and then you have Saxons, Franks and Huns for all sorts of aggressive horde style armies. Picts for anything that likes a nice forest to hang out in.