- We will order 500 units of each model.
Your first sentence is not impossible if you add an "of".
- We will order 500 units of each of the models. (Note that "each of" requires the plural noun.)
I would like to point out that, to me, "each of the models" makes it sound like you are talking about people (ie catwalk/fashion models, Kate Moss, etc), whereas "each model" would lead me to assume you were talking about some kind of equipment.