Sound system
I installed a sound system on my Hunter 30 about 3 seasons back. I just spliced into my cabin light circut and had never had any problems with power/ blowing breakers ect. I also have 1 hella fan wired to the same circut. While i never turned every light on the boat on at the same, and the fan, and the stereo, I doubt it would trip breaker as all of my cabin lights have been switched over to LED's. I would lag into a current circut and not sweat it.
Make sure you get a model with a REMOTE!!!! I didn't(it was only 20 dollars more) and I am still kicking myself. Unless the stereo will be out in the cockpit, buy automotive as the prices are better, features are better, and they last just fine. Most models will now control 2 sets of speakers or more, I split front/back speakers between cockpit and cabin, so we can still listen while kids sleep/watch movies below ect.
Buy marine speakers for outside for sure. This is where to spend the cash, don't skimp. Don't get caught up on speaker size too much. You'll be surprised how little power you need as you are so close to the sound and usually want to converse while sailing. Speakers for the cabin can be auto as they will not be exposed to spray,( I hope). Make sure that the cockpit speaker are either sheilded or far enough away from the compass so you don't end up on the rocks or the beach.
Pray for Wind,
Lugeman