Buying a used Prius, what should I look for?

There are four Prii I'm eyeing currently, all in the same ballpark price range. They are:

2006 Base 80k miles ($10k)
2008 Base 70k miles with package #5 ($13k)
2008 "Touring" 80k miles ($13k)
2008 Base 30k miles Certified Used ($15k)

How important is mileage on a Prius? The 30k miles one is a two-owner car, and the second owner (who had it for ~4 years) had a suspiciously low miles/year rate of under 3000 miles per year.

Is there anything I should look for when I go onto the lot and inspect the cars themselves, or anything to look out for in the service history? I've already seen the first Prius in person (2006 Base with 80k miles) and one thing that concerns me is that there's significant rust on the driver's seat track. Is this expected for a Prius that's been driven in an area with a lot of snow and salt? (Buffalo, NY) How susceptible is the Prius to rust in general? The third Prius (2008 "Touring") was bought and driven in Florida for 95% of its life, is that something to take into consideration? I live in Buffalo, NY.

 

Thanks for responding. Are there any other good places to look for rust? Or any other signs that the car may be a potential lemon?

Also, I received an Autocheck from the dealer (independent dealer, not affiliated with Toyota) for the Florida car, and the last entry is "REPORTED AT AUTO AUCTION" from just last month on December 17. I figured that's how it found its way up here in NY from Florida. How concerned should I be about this?

I'm also going to take the car to get it inspected, would it be a good idea to take it to the local Toyota dealer's service department? I figure they'll have more experience working with hybrids than the average auto shop.