electrical wiring help PLEASE

Hi All

This is my first wiring project so I thought I would put it up here for some advise. At the back of my car I have a 175 Amp Anderson plug, this is used to plug the 80 amp Christies charger into to, to charge the car batteries. I wish to run power back to our boat so we can charge the batteries as we drive and also via the 25 amp charger mounted in the car when stationary. I have three batteries in the boat, 2 x 50 amp AGM Excide orbital’s wired parallel, these start the motor and provide power to accessory sockets in the boat, these are located at the very rear of the boat. The other is a 100 amp Gel cell under the consol in a purpose made battery box with 60 amp fuse and is plugged in via a 50 amp Anderson plug, it purpose is to power our Minn Kota trolling motor.
What I am considering doing is 175 amp Anderson to 175 Anderson lead, this will go to a storage compartment in the front of the boat, this lead will be removable when using boat, at the storage compartment the lead will plug into another 175 Anderson which will split in two(the ones for sale on eBay actually split into 3 x 50 amp plugs, is this the way to go?), one lead taking charge to the rear orbital’s, the other lead to attach the battery box 50 amp plug. Allowing for slack, the lead from the rear of the car will be 3 or 4 meters in length, the other two approximately 3 meters. Unsure of which wire size to use, is 6 B & S the go and will this fit into 50 amp plugs ok, will this and plugs be sufficient for charge from alternator (78 series T/D troopy), will 13 mm solid tube flexible conduit (the black tube stuff, not the split) be large enough to take both wires easily? Is there a way to crimp the plugs without the correct tool, I seem to remember seeing the 175 amp tool and it was large. Maybe I could get an auto elect to crimp the two 175 amps and do the 50 amps myself? Re the wire does it matter if it is twin? I have seen that Anderson plugs from eBay are 1/3 the price of what they are at Jaycar, anyone know where a good place to source wire is? Have been quote $15 a meter for twin 6 B&S from auto elect in Darwin.
Hope this makes some sense?
Cheers Lyndon