Buick century 2000

Looking to buy a 2000 buick century from a buddy. Test drove it yesterday and had a bit of an issue. When I accelerated there would be a heavy vibration and it would worsen trying to go up a hill or accelerating heavily. It would also stop accelerating if I pushed it to the floor or pushing too hard going up a hill. No vibration when coasting or letting off accelerator. There was a code for o2 sensor and exhaust recirculation sensor/system. Haven't taken a look at it other than test driving and popping the hood. Don't mind trying to fix it myself but just trying to rule out if it's a major/ expensive part issue.