Need Some Help With My 99 Es300 Lexus

Good day to you sir, I was hoping I could get some help with some issues for my 99 ES300. I recently bought this vehicle and have now had it maybe 5-6 months total. It has 140K miles on it currently which it had 130K+ when I bought it. Anyway, I have two codes popping up after diagnostic test and they are: P0446 & P0420. Also it won't always engage in gear. It almost feels like being in neutral while in gear. I have not tried anything as of yet due to not being sure what the right direction to go in would be. The codes I've gotten are P0446 and P0420, I was told to replace the gas cap, and P0420 is showing to be an 02 sensor with a possibility of it being catalytic converter. Also, I have gotten in the car ready to take off and throw it into reverse and it does not engage in gear, it just sits as though it is in neutral. Give it some gas and the RPM's shoot up and everything but there is no power to the vehicle until I pushed the gas further and it would kind of forcefully go into gear. Also I have been driving down the road and it would just become none responsive and slowly come to a stop while still in drive. I am hoping it is not the transmission slipping as I have only had the vehicle 5 months/10,000 miles. I would greatly appreciate any ideas or suggestions as to what this actually is as I cannot afford to replace each thing if it will indeed not fix the problem. At the moment I am between replacing the 02Sensor/Catalytic converter or finding a new transmission. Please help..