the wireing diagram is almost perfect, you only run the negative to the relays the lights already have a suitable earth, earth is not the problem area, otherwise yep spot on
Yes 30amp 12 volt is perfec,t I run a 20amp large blade style (on the left in picture) fuse at the battery end so if you ever blow a fuse it is easy to get to I was nearly tempted to buy the relays that have piggyback fuse but then you would have to lift the tank if you blew one!
I used 25/30 amp dual core (inside an extra cover tubeing) for the power supply + & -- it's 4mm not sure of the conversion you will need about 3 foot but allow a bit extra when you buy it just to make sure, you need to cut a 6 inches of and slice it in 6inches in from the end
You will also need 3mm 15/20amp dual core from relay to harness you will need 3 pieces about 14 inches long
you also need shrink wrap tubing and some insulation tape also two eye type connecters to connect the wires to battery if you find your battery terminal bolts are two short buys some m6 stainless steel bolts another thing I use and should be available at you Auto spares shop is electrical grease use it on all connecters and where ever possible solder all joints it is even a good idea to put some solder on wires you are going to crimp and use a good crimper.
There I think I have covered everthing I will be making one of the harness up this weekend and take pictures for those who are not sure about what I mean about splicing the power wires