U need to explain to us what your planning on doing and post some pics. This looks interesting.

The old PC power supply's are going to be horribly inefficient and draw a fair amount of power from the wall for the feeble output they have

have you figured out what the system is really going to draw? not what it could draw but what i will take when playing at the volume your going to play it at most of the time

as long as you can provide slightly more power into the buffer battery then that you should be fine

i used to run my shop setup on a single group 65 800cca battery and 10A charger
it worked great for my pioneer HU pushing a 50x4 mtx amp and a tsunami db1100D running at 2ohm
could make the whole tin clad shop rattle lol (28'x36')