The Prius is extremely reliable. There's not a lot of issues with them, and even the hybrid batteries will probably last the life of the car. Generally you're better off having a Toyota dealership do the service, but the need for service is so infrequent it's really a non-issue. I'd feel very confident buying a used Prius with 50K-100K miles on it, but you might want to be cautious with the 1st gen models (2001-2003) if they've got a lot of miles and still have the original battery.
And the type of commute you describe is ideal for the Prius. Once you learn how to use the pulse and glide driving technique you'll probably average well above the EPA standards. I get about 50 MPG in my 2008 Prius, and most of my driving is short trips around town. I always get better mileage on the highway.