Yeah the main problem with the Seleucids is lack of range and how spread out their empire is. Expect within 10-15 turns to be at war with Parthia, Pontus, Armenia and Egypt. Taking Egypt is good, but I also like the idea of taking Pontus. They're easier to get rid of and fairly quickly you can own most of asia minor, which should be easy to defend.

Problem is Seleucid is an infantry heavy faction (with some decent cav and specials but almost no range) surrounded by enemies who are either cav heavy (Pontus, Parthia, Armenia) or archer heavy (Parthia, Egypt).