Keep the fuses as near the battery as you can, as said the k3 will control the fans, though you can have amanual overide if you want, last thing you want to be doing when racing is watching a guage for temp, let the ecu do the work, set at what temps you want.
Most wire will be ok with 16/020(thinwall) which carries 11 amps continuous, or next one 32/020 carries 16.5 amps continuous, solenoids as said do take a whack of current to move but much less to keep it there, so wouldn't bother with fuse either.
I've got 200 metres of black and red 16/020, and 32/020 as well as 28/030(25amps)
tabetha