Auto Parts: Buying a 2001 toyota corolla, toyota corolla, o2 sensors

Question
Hi

I am planning on buying a used 2001 toyota corolla basic model. It has 88000 miles on it. I did a carfax on the VIN and found that the second owner had the car since 2002, and drove only 45k miles in 6 years and suddenly drove 27k in the last 9 months since april 2008.
Anyways, I checked out the vehicle. It had some scratches on one side but nonetheless seems ok for a 2001 model. Now comes the problem part:

The guy who has the car says that its his friend's girlfriend's car [too many people in the loop] and he sold it to him as he needed cash. Now he is selling it off for quite a good price actually, that's why i am interested and writing this here. Now he says that the only problem with the car is that the O2 sensor needs to be replaced, he said that he did a diagnostic scan on the car.

Thats fine with me, I read that O2 sensors tend to wear out and need to be replaced. My concern is whether anything else is wrong with the car.

I am going to go and have a final look at the car, along with a mechanic. So what I need to know from you guys over here is :
What should I be concerned about?
Which other part needs to be checked?
Whether the catalytic converter can be bad? If yes, how do i get to know that?

The car as such drives fine, but still I am not sure whether there can a problem afterwards. Although I am taking a mechanic along with me, when I go and see the car, but still I want to know from your experience, what else can be wrong with the car.

Any help from you guys is sincerely appreciated.

Thanks.

Answer
Check the brakes for wear, make sure all the electrical items work properly, check tires for uneven wear or just wear in general, and finally DRIVE the car.  Sometimes simple things like worn brake pads or ball joints aren't that big of a deal, but if you come up with a large list of things, that's where it can get spendy and offset the price of your "good deal".  :)