I think its the OPs decision as to how much he wishes to spend. It is certainly a cheap fix - but good - here I think we differ.
I dont think you can compare it with brakes or steering, but a prospective fire, with children on board is not something I would like to see, particularly as the cost is not, necessarily, much more.
I think the same applies to all vehicle work. Only do work you feel competent and confident to do. Do not cut corners unless you are aware of the risks and consequences involved.
My understanding was that the OP did not want just straight replacement wiring. he mentioned power to the rear, a rea work light, etc. These are all things that need to be catered for, not just added on to existing wiring.
Once the OP has decided on all the things he wants, or prospectively wants, then the new fuse box he has purchased needs to be sorted as to what fuse rating to use for what circuits. That may well have an effect on the current rating of wires required.
You wouldnt rebuild the brakes on a car without knowing what the vehicle was being used for. The same applies to anything that is a safety critical component.