How close to a realistic Seleukid army would this be?

[palmtree]

Stone walls are the best deterrant for all nomadic factions, since they're not bringing field artillery and all the horses will have to go through a gate. If you can destory their ram, you're all set. If you can't, two units of pantodapoi wll do the trick.

There's usually some mercenery infantry tagging along, but there shouldn't be too many of those.