Brown/Black in oil fill area? Do I walk on deal?

I have filled out the contract for a 2003 Prius. However they don't have the cash yet. Car is fairly high mileage, about 100,000 but the Longo Toyota service advisor said he would choose this 2003 car over a 2001 with only 55,000 I was looking at. This car is immaculate compared to the other two cars I had narrowed the field to. At least inside, outside and in the engine compartment. However, the very last thing I checked on all the final candidate cars was looking into the fluid chambers. And when I removed the oil fill cap on this one it was quite brown black (burn) compared to gold or nearly clean on the other two cars. I could scrape some of it off with my fingernail.

The dealer offered to give it an engine flush. But I opted to contact the Toyota Service desk first. I am bringing it in tomorrow to Toyota for a check which the selling dealer willingly agreed to. I planned to bring the car in for a check anyway...but I decided against it because the car was so clean and under a dealer warranty and it was the highest priced of the three I was considering. I thought I could get away with it in this case.

I can say the car drives just beautifully. Everything else works like a new car should. Brakes are awesome, it shifts smoothly. The engine feels sound as a pound. Toyota says they don't do engine flush on a Prius or most of their cars. She says they tend to require very little service and if there is a sludge Toyota would do a full engine rebuild on it to get everything out which she said would be very expensive.

The oil looks very clean but I imagine that doesn't matter.
Obviously I am going to put the most weight on the checkup tomorrow. But I imagine there are some experienced Prius geniuses here. I would like to get your opinions on the off chance Toyota service is a place where they like to be thorough maybe above and beyond the call of duty.

 

I would say there was one Prius I looked at this morning where I thought I might have felt the transmission transitioning up and down. But I did not buy it for a number of other 'warning' signs.
This car feels very smooth. I will take it for a ride again tomorrow and watch for that. But my memory is the car feels impressive and buttery. I am not a Prius expert though. I have only been introduced to them this week. So I thank you for the tip.

 

Thank you. I very much appreciate all thoughtful responses I get tonight.
This one was comforting as I very much want the car.

 

Just for closure on this thread here's the follow up. I took it to the Toyota dealer. He didn't think it was sludge but couldn't be 100 percent sure without taking it apart. But he was mostly sure it was okay. I went with that. We did an engine flush and I bought it. Looks clean there now of course. Car drives great.