Good luck with your J/30 quest. It's a good choice for multiple uses, can provide many years of family fun and being a fiberglass boat, is nearly infinitely repairable if you are willing to do the work yourself.
The alternator might be your easiest fix. The 2-pin field winding connector can become intermittent. The terminals in this connector might be made of plated brass instead of plated phosphor bronze, which means that multiple un-mate and mate cycles can cause them to open up and provide less pressure against their mate. That was the case on my boat. Running the engine with a voltmeter connected to the system (large terminal on the starter, the alternator output, the battery positive, etc.) is the first step, then again while moving the connector to see if you can get the battery voltage to jump up. You might be able to fix it by somehow bending them back a little bit. Without fixing it, even a new alternator isn't going to work. If it isn't the connector, it could be the diodes which can become blown if someone shuts off the battery switch or turns the key off in certain situations before stopping the engine. These can be checked with the diode check range of a multimeter if the alternator is apart. If the alternator is still in the boat and only 1 or 2 of the 3 diodes are bad and you have access to an oscilloscope, this can be determined by looking for continuous train of small voltage peaks with an oscilloscope connected to the output terminal. Should the brushes be bad, even those can be replaced.
With regards to leaks and wet core, all of these areas should be addressed. Deck hardware mounting points, (especially those that have high loads), exhaust port and chainplate exit slots are likely spots for water ingress and eventual rot/delamination. They should be checked by tapping with a small ball peen hammer for delamination and at least epoxy isolated to prevent further problems. Those that are not yet delaminated, but you don't quite have time for, should be put on a long term list for repair. Wet deck is fairly easily cut away, ground out to a 12:1 scarf (taper), recored, replaced with new glass and non-skid. Some of the deck mounting hardware problems can be easily seen by discoloration and cracking from the inside of the boat.