imprezawagonchica

I need some serious help.....I just got an 07 Subaru Impreza wagon due to wrecking my Tribeca B9. It has not even been a month and already pure hell. I bought off someone a state away so no warranty. Anyways what happened at first is it threw codes po301, 303 and po420 so first I replaced both o2sensors that cleared up the po420 code. So now I have both 301 and 303 codes. So I just started small today and replaced pcv valve made sure no hoses rotted and tight. Well now it wont start the car stalls right away if i hold gas it backfires. then threw code po103 and 113 but no 301 or 303. Anyways I am clueless on what to do and why it wont even run now.I do have an ignition coil pack on its way just wanted to do this since I had the part. I tried disconnecting the MAF sensor just to see if it would atleast start but no luck. I had read another form that said to disconnect and see if any change. I really need help I have already spent $600 in other repairs to the car and I only have about $300 left to spend till I am flat broke. (Oh and MAF sensor is clean no crud)