Theres not enough info to answer all of your questions, but I will try to answer what I can.
There really isn't an amp kit for marine installs like you would buy for a car amp install. They typically come with a long run of B+ and a 2 ft section of GND. The reason being is that cars are one big ground plane and boats need to have all the amp grounds run back to the battery, same as the B+. I recommend purchasing your length of cables in bulk.
What kind of switch is currently installed in the boat? "Perko" is only but one manufacturer of marine battery switches and they have a few different styles for different purposes. If the boat only came with a single battery, its a good bet that the switch is only an ON/OFF. What you need is a dual-battery switch.
If you go with a dual battery setup and a traditional dual-battery switch, all but the auto-bilge B+, if the boat has an auto bilge, will, be relocated from the original battery's B+ to the common post of the new switch. providing you do not need to lengthen or replace the original cables, then that would leave you to build/buy a new cable from the cranking B+ to the switch, new house battery B+ to the switch, and one from original battery GND post to the new house battery GND post. 1/0ga would be a safe cable gauge for those battery cables since one and maybe both will be used as a cranking battery.
Relocating the original cranking will most likely require upgrading the cable size as well as running longer cables. I would also look at leaving the battery where it is, placing the new battery and switch close by and then running the proprietorial gauge cable for the amps.
Cant beat the price per battery Ah of a group-24 marine deep-cycle.