Audio is the operative word here. Audio requirements are not anywhere near the current requirements of EV use.
I'd beware of that guy - 'used' batteries are probably OK for testing if someone gave them to you, but don't expect more than a year (if that) out of batteries that have been used by various people for unknown audio applications (were they used for a year, always to 100% depth of discharge providing sound for tailgate parties? There is a limit to the number of discharge cycles of any battery - Why were they removed if they weren't bad?) Why spend half of what new batteries would cost on questionable reliability?
Audio cable does not have heat-rated insulation. If you get it hot (400 or so amps on #4 cable will get it hot) it will probably smoke. Get real cable with a temperature rating stamped on it, not see-thru plastic. In any case, you probably want at least 1/0 cable, probably larger.
Get a single battery charger for the voltage you intend to use. Multiple chargers are a genuine headache - don't go there! Imagine every night after you use the car, finding a bunch of outlets, hooking up battery clamps to all the batteries, and hoping you don't blow a fuse because you don't have enough current available in your garage for all those 100 amp chargers. Then going out in four hours and hooking up the rest of the set, and doing it again four hours later until all the batteries are charged... every day... If you want to use the car, make it simple AND reliable. That will help make it enjoyable.
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