Intake manifold

I have a 1996 Chevy Tahoe with 120,000miles 5.7L
In August I got a new intake manifold gasket. My check engine light came on last week and it said O2 sensor bank 1, I replaced that along with new spark plugs, wires, caps and my check engine light stills comes on now it says O2 Sensor bank 2, plus a misfire on spark plug, cylinder 2. I paid $600 for the manifold gasket and I brought my truck to
another mechanic and he told me that he needs to remove the intake gasket and put a marginal fuel sender, which is going to cost another $600. He told me that he thinks the previous mechanic did not hook everything back up the way it is supposed to be. I am a woman and I think I am being taken advantage of. My car sounds loud when you turn it on and when I stop at a light it feels like it is going to cut off.

Thank You For Your Time And God Bless You.