95 Previa, Do I Need Iav Or O2 Sensor?, 95 Previa idles high when warm Options

[Tutman]

The check engine light came on my 95 Previa (98000 miles) and it started idling high once engine was warmed up.
I took it to my mechanic who put it on the diagnostic computer and said it showed "lean mixture".
He cleaned the mass airflow sensor and the light went out. On the way home, the light came back on. After 2 hours of searching and tweeking, he said he "thinks" it's the Idle Air Control Valve becuase when the engine is revved, it the rpm's do not come back down all the way. He put a carb jet in one of the vacume hoses (to restrict air flow) and it fixed the idle high and rev problem, but the check engine light is still on and it idles uneven in nutral when warm, but idles fine when in gear.
He said it was only a temp fix until I can afford a IAV.
Ever since then, the idle has gotten progressively lower when started cold, but only until the engine warms up. Now it won't idle when started cold without giving gas peddal until it warms up a bit.
He's not 100% sure about exactly what part it needs and I don't have the $$$ to start buying parts that won't solve the problem.
Any help is greatly appreciated!