$650? Sounds like a deal, mate. Boat looks pretty good and you got a solid-section mast and the optional cooler rack! I'd check the rust on the trailer (looks like mostly surface) very carefully, especially around the springs (looks a little worse), axle and hubs.

For the repair, you can buy gelcoat in a tube ready to go - just in white, though I think. That would be where I'd start. There are some experts on this board, however, that can advise you far better. I don't think the Bondo is a good idea.

Happy sailing, bud - put up some pics of your sails when you can.