I own a Melges 24, and while I cannot compare it to a J80 for you, I can answer a few of your other querstions.
We sail PHRF with it in Portsmouth NH. As there were no suitable facilities to dry sail the boat in our area, we painted the bottom and have left it in the water all summer.
The ideal crew size is four people, although on nights with very light wind we will race with three people. We even did a double-handed race with just two, and that went pretty well. If you are going to fly the
spinnaker, at least one of you crew members will need to be pretty strong to handle the chute. There are no winched on a Melges 24, only ratchet blocks.
The claim on the Melges is that you can set it up with practice and have it in the water in 30 minutes. NO WAY! Unless you have a facility where you can leave the boat rigged and just drop it in the water with a crane, plan on AT LEAST an hour to set up and tear down. Also, putting up the mast requires a LOT of muscle, at least three people, and two of them very strong.
That being said, the boat is a real pleasure to sail, and we have been competitive within our fleet. Our rating of 102 puts us in the same group as boats in the 33-44 foot range, so we spend a lot of time looking for clean air. It is also a very comfortable boat to take people out on a nice day sail, with its big, wide cockpit.