Spiggy

Hi all I have a wiring loom for my mk4 spit already to go in, got it about 7 years ago, but 2 little ones put that on hold for a while just starting to sort it out now started the rusty bits of bodywork, next will be the electrics. Im thinking of upgrading the fuse box as never been happy with just the 3 fuses and putting the loom in would be the ideal time to do it. Ive never done much with car electrics before mostly sorting dodgy earths, but can follow wiring diagrams Im a service engineer so used to wiring up comms and data cables from the diagrams. The main problem Im concerned about is working out which fuse to put in the circuit Imunderstand the maths of ohms law to work it out, but how do I know how much power each circuit uses. Is it a case of using a multimeter to measure the current, or is it written down somewhere. Also my car battery has had it, is it worth upgrading the battery and alternator currently has a 17acr Ive seen you could put an 18acr in with some modification on wiring. thanks in advance for any help anyone can offer Ian