I would go with the brand new tennis tutor plus w/ 2 year warranty. If your used and out-of-warranty ball machine dies in a week, you have an expensive boat anchor. Yeah, you could get a repair quote from Sports Tutor, but who knows how much that will be?
By the way, if it absolutely must fit in your trunk, have you looked at the Tennis Tutor Prolite? I have one, and I think it would fit in the smallest trunks. You do lose all the good features of the Plus, though, like topspin and backspin