Brand of oil would not have made my list, even long list, of things to consider - or to not buy the car if its had Penzoil or Quaker State. Around here those are pushing $2 a quart and are considered premium brands. There are 117 motors out there that have gone hundreds of thousands of miles on such oil.

I think there's a simplier question - i.e. are you going to work on the car yourself? If you are going to work on it yourself, and enjoy it, the mechanical condition of the car is less important than the cosmetics - paint and interior. If you're not going to work on it yourself, I'd say you shouldn't be buying it. It's a 15 year old car that needs regular maintenance and repair. Parts have gotten quite expensive with the value of the dollar and labor continues to climb. These can be very expensive cars to maintain when you turn the job over to someone else. They really aren't viable transportation compared to a newer car. Maintaining them is especially expensive in the context of the market value of the car which is steadily marching towards nil.

I think it's catch 22 on a 15 year old Mercedes these days - if you don't know the car you should get a PPI. But if you need a PPI you really shouldn't be buying it.