No love for Skar on this forum really, or most caraudio forums lol, I use some Skar, performs well for the small dollars it costs.
Personally I would go with 2 12s over 4 8s.
Amp depends on the subs, how much power at what impedance. Skar sells plenty of sub amps, apex audio makes decent stuff that is budget friendly too.
4 gauge power and ground, should cover you for most power applications, stock alt will be fine, good agm battery is always nice to have.