see inserted comments.
good idea.
thats a start. kit is good for that amp.
no. it will be enough for the 4ch you have only. if you are doing two I would suggest 4 awg and a 4-8awg distro block at the other end
correct.
it can. when you start getting REALLY big then is becomes a concern.
how big is the fuse(s) on the current 4ch that is there. if they are BELOW the 60A rating leave it be.
no probs. send cheque to address in sig