first ballast. you can replace the rear bags with anything larger. if you have the old sprinkler valve setup, you have 1 hose in and 1 hose out and a vent. pretty straightforward. my old launch20 is VERY similar to your lsv and we went with 1100's in the rears. they don't quite fill up 100% before running out of room to expand so we found them to be the best choice to maximize the ballast. on that boat when we only had 4 peeps, we ran listed, not fully weighted with a suckgate as they weren't around yet. 1100's in the rear surf corner, factory 400 in the center ski locker. we put a 400lb sac in the bow and ran a 400lb sac on the surf side seat corner. that gave a good wave. rubrail was at the water surface. down side is it takes 15 or 20 to swap sides with the old sprinkler valve setup. we were going to add dual drain/fills but never did. it is pretty much plug/play but there are a few differences in connectors from bag to bag. I always tried to stick with fly-hi.
pm for more info if you like but sure others that have/had owned lsv's will chime in.

stereo, need more details. my launch20 was a simple headunit that we replaced with a newer kenwood unit. not knowing what you have and no pics make it hard to talk about. I would assume you have a simple deck, speakers and maybe an amp or 2.

I won't touch the led question as we never put any in the 4 boats we owned over the years. never saw a need for them. rather spend the money on boards or audio.

welcome and post up a pic of the whip....