Yes, use spies and more spies. You should be able to quickly balance your budget, especially if you give NIsa the mine resource it should have. I managed to keep at least one spie in almost every eastern Seleukid city and they had to constantly shuffle troops around to keep the peace and thus couldn't focus on me. And as revolts started occurring next to my territories I would swoop in and take the rebel settlements. After a while you can outright buy a Selukid province that is in revolt from the AS for roughly 7-10k a pop. Using this strategem, and with the timely declaration of war between the Baktrians and AS, I managed to keep the peace with the Seleukids for almost 40 years, and allowed to me to thoroughly crush the Baktrians when the AS requested military aid from me. The AS only declared war after I finally took Persepolis when it revolted for the 4th or 5th time.
Rather than buying Pez mercs, stick with the Pantodopai Phalangitai that will be available almost everywhere in Regional MIC 2. They are more sturdy than the infantry you'll get faction-wise, but the Iranian Archer-Spearmen are some awesome support troops, as well as Shuban Fradashkana. Your basic HA will carry you a LONG ways, and you have access to several good melee cavs of medium and high quality, so there is no cavalry or infantry threat that can't be dealt with. I broke Baktria's Pezheteraioi line with Pahlava Shivatir and Denbed Asavara, with only 3 Pant Phalangs to distract them (I don't believe they actually locked horns with the Pez, but I know they tussled briefly with Baktria's light inf and archers when the AI got desperate).