You need to find out what year boat it is and knock/push on the hulls to see how solid they are. Good hulls ring, OK hulls are more of a thud and bad hulls crunch when you push on them. The year just tells you about the construction details.
For $500 with trailer it's hard to go too wrong, but if the boat truly is junk then it can still be a little bit upside down. If the hulls are solid or only have small soft spots then you're OK even if you have to buy some used sails.
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'00 H16 #104691
'78 H16 #32692 ex-rental [gone]
Old Holsclaw trailer
My Hobie 16 pages