Your amp prolly puts out around 8-900 watts tops, so I wouldn't be concerned with the car electrical unless you plan to run over 2k watts rms.
Yes bespoke has RCA out, you also have sub level control through it. If that isn't enough, just pick this up they work good.
http://www.sonicelectronix.com/item_...xess-AALC.html
As for the wire size, a real 4g wire would be fine, If you want to play it safe for anything in the future tho sure, go 0g.