I'm not so sure how well a Greek phalanx would do. However, if you want to look at an ancient army that would do well against early medieval armies look to the Romans. I know Dan Carlin from hardcore history, has made the argument that a Roman legion from the late republic/early imperial period...
Coins can also serve as tools of propaganda. Taking this path you could use the generic gold and silver, but distinguish your coins by what is on them.