2001 Toyota Corolla multiple problems

1st engine light on code po420 I replaced the 02 sensor now the car hesitates and like in the mornings and after sitting at work all day when I try to take off it sits there for a few seconds then takes off? Also the mass air flow sensor gets gunky? When I step on the gas it kinda sounds like a vac leak? I was told the cad conv and the mass air flow sensor? I have alot of black smoke, burns oil, running real rich. Also I could smell gas under the hood before replacing the 02 sensor now that smell of gas is gone under the hood but I feel like I smell it on me when I get out of the car? What all is wrong with my car and what should I do to fix it? Thank you so very much