If you are going WRND then the first black box you looked at will be perfect. As mentioned it is a nice piece of kit.

You need a take up from the crank battery and please investigate what the guys are saying about the D4. I know the electrical systems are finicky.

16mm multi core or multi core welding cable would be a good choice but you can probably use less mm2 with the DC-DC. WRND will be able to recommend what cable will be best suited, usually 4mm2 or 6mm2. You will need a good in line fuse. NL do a good one you can buy separately from any 4x4 shop and you can get the spare fuses for them.

After the WRND you can feed the black box via the red 50 amp input or direct to battery.

Now you can decide what battery you want as the WRND will not limit you as the solenoid would and should feed the 14.4 minimum that a deep cycle will require.