Apart from the basic kit that has already been mentioned,a toilet roll is also high up on my list of what i would consider a basic started kit.
When i am away for the whole day or weekend then the gear i carry in the boot of my truck is always full of detecting gear,why i carry so much is beyond me,but saying that when you are say on a dig miles away then i will in theory never get stuck.While detecting then my basic equipment is just spade,finds pouch with basic spares in like coil bolts,spare batteries for detector/s and pinpointer and a very basic emergency kit and phone.
Spades i always carry 3 from my everyday one that has lasted me possibly 10 years and also really heavy duty ones for when the ground is like concrete,you could drive a tank over them and they still wont break.
Clothing i think is also important and usually carry 2 complete waterproof and a backup lightweight outfit for during the summer months,headphones are also in my mind essential and carry with me when ever i go out 2 the WS5 full cup for the Deus,full cup for the Nox and also another spare full setup that i use on a standard 1/4' detector jack and also the XP wired backphones as spares and also summer months.
TDI Pro,T2,Nox,Deus,Nexus MP,Nexus SE,Mirage PI,Crossbow PI,DFX,Fisher TW-5 twin box,Arado120b and others.