Hi Rebecca and welcome to the forum.
So you have an eye on a Toyota Hybrid, a Prius or an Auris. Funnily enough I was in the same position last May. I researched the cars, found lots of useful info both here in Uk and US. Some Hybrids, usually the Prius (not sure whether the Auris is released in US) have done 250k miles and still look and drive well (according to the owners). For your info (just in case you didnt know it) both the Prius and Auris have the same Hybrid parts in - ie 1800cc petrol engine, a large battery pack under the rear seat (which owners dont need to touch) and a CVT "gearbox. If the "big" battery is charged you can drive up to about 20mph for about a mile on EV electric vehicle. Doesnt seem much but these cars are not wholly battery driven that need to be plugged in to charge up. Hybrid technology is petrol engine plus battery - at times the revolution of the wheels and braking charges the big battery up. Today I been cruising along a flat road at 40mph and the car showed it was doing its max on the gauge of 100mpg, so the battery is assisting the drive, petrol engine being used lightly. The CVT transmission is really good. You have D for drive, R for reverse, B for assisted engine braking when going down steep hills, n for neutral and P for park. For most driving it D for drive, no gear changing and unlike majority of automatic gearboxes there are no gear changes taking place automatically - its stepless and it is so smooth, and if like me, you will wonder why you didnt find Toyota Hybrids sooner.
Prius or Hybrid, as I said the Hybrid system is common to both models. The Auris body is shared with petrol and diesel systems as well as Hybrid, the Prius is ONLY available in Hybrid form and is designed to slip through the air with little drag, so gets very good mpg, a bit more then the Auris even though the Auris being smaller will be lighter. The Prius costs more new, but both keeps there prices well, so year for year, equal condition, model, miles etc you will find Prius to be more expensive used - I was looking around the £7k range and the Prius was about £2.5k more. You get more boot space in the Prius and more rear passenger legroom (its huge in the Prius), so its your choice which you go for.
High mileage vehicles. I not too afraid going for higher lies if the price is set accordingly. What I do look for on any car, and especially high mileage is service stamps in the vehicle service book, stamps (services) on time, and preferably main dealer stamps, but other dealer stamps are ok. With the car you looking at £180k+, it says 15 service stamps. This should be 17 (every 10k) but 15 not bad. Check to see if they done by Mr Toyota. I not heard any problems with the engines, and my research did not pull up problems with the main battery. Good job because they cost approx £4,000 to replace. But no where did I read any advert for an Auris nor prius that said "new Hybrid battery fitted", and you would definitely promote that if you were selling wouldnt you - I know I would. You just dont get problems with them. In fact for £45 Toyota will check the Hybrid battery. - well worth it.
The dealer selling the black Auris is also selling a white one with 150k miles for about £800 more. I hate black cars, get dirty so quick, I would buy white over black any day and road dirt doesnt show up as much on white as it does on black paintwork (road dirt is grey) In fact my fav colour for a car is silver - its nearer to grey and I dont wash my car for weeks and it still looks pretty good - maybe brush wash the wheels - but, hey, thats a personal thing.
As for the rest of the ccar (Auris or Prius) the interiors seem to last well, and all the other mechanics ie brakes, tyres, exhausts, etc are to be viewed as any other car, they need checking by any perspective purchaser. A long MOT is a good thing (both the black and white ones have Jan 19 Mot - good).
I would go and test drive both cars, and if there is a Prius for sale within your travel range go and test that as well just to try the driving experience. My wife has an Auris Hybrid (8 years old like my Prius but only 67k miles as compared with my Prius 107k) and I prefer to drive the Prius. I notice both the black and the white Auris for sale have 15 inch wheels (quality tyres on them), both have kerbing scuffs (talk down the price). Alternative is 17" wheels. 17" tyres tend to give a harder ride but more stable on fast bends, 15" wheels give a softer ride because there is more side wall that flexes and adds to the "suspension" effect.
Happy hunting, and do have a look at slightly older Prius cars, say 05-08 reg. These will be Gen 2 models which have a 1500 engine but basically the same Hybrid effect with CVT transmission. I have seen 05-08 models with 50,000-70,000 miles around the £4k mark, and still looking VERY good. Prius do hold their price.
Added - try not to do a part exchange. If you can go in with no p/ex and cash you could come away with a bargain. Buy privately and you could end up with a better bargain - but no warranty. Choices....!!!!