The Seleucid do have it tough. To add to the above I like to hire Mercs for the initial push into Egypt. I leave the Eastern Inf (or whatever they're called)alone but hire camel units and Greeks when I can. I keep a diplomat around Babylon and bribe any small group of Parthians I see. It might cost more over time but if they show up with a large army led by a faction leader, money won't help you. I also make lots of diplomats to spread all over the world selling alliances, trade rights, maps, etc, to keep the cash flowing.