The two most popular are the Little Giant (L/G) or the Hova-bator. Most people prefer the Hova-bator. We use an L/G with great success. You will want one with a fan to help with more constant temperature control. I would highly suggest a turner also. It saves a bunch of work and headaches. With a turner, you won't have to be opening your bator every few hours to turn the eggs by hand and loosing your heat and humidity.
I would certainly ask about general condition, how many hatches they ran through it, what was their success rate, temperature regulation, any problems they had while using it. If they only used it once and had a terrible hatch then you have to take it into consideration that maybe something went wrong that was of human error.
There are plenty of bators out there with turners that you can get used for $50 to $75. We sell new L/G's with fan and turner for $114.45 plus shipping so that gives you a price comparison.