Car Sputtering

I have a 1991 Ford Mustang 2.3. When I start the car, it turns over fine. It even idles fine for a few minutes, but then begins to sputter like it's out of gas. When I step on the accelerator, it hesitates and sputter's even more. I have to let it warm up for 5 minutes until it clears whatever is wrong with it. If I drive with it still sputtering like this, it won't accelerate, it just spits and sputters even more. I had Aamco run a diagnostic on it about 1 month ago, and they told me I need to have the O2 Sensor replaced. They want to charge me around $500 for an $80 MAX part. Should I buy the part and replace it myself, or just suck it up and let Aamco rip me a new one?