OK, I've a spare 5 mins, I'm going to try and list everything that gets carried here, and this nothing compared to some!
Tools-
Fairly Std. toolset - multiples of 17s and 19s, club hammer, screwdrivers, allen keys, etc. Look at every tool you use when in the workshop, and make sure you have one with you.
Spanner big enough for rack locknuts.
Stilson wrench.
Sledge hammer.
Jack
Pressure gauge
Electrics -
Multimeter
Spare plug leads, coil, dizzy cap, sparkplugs, ignition box
Cable
Cable ties
Crimp Connectors
Insulating tape
Cutters and crimp tools
(scouring eBay for a good timing light)
Spares -
Jerrycan fuel
Jerrycan water
5L Engine oil
5L Gear oil
Hubs
Coilovers
Lower arms
Balljoints
Fanbelt
Brake flexi hose.
Rose joints all sizes
Steering arms and rack
Halfshafts
Diff
Propshaft
Front and rear bumpers
Assorted bolts and nuts
Safety -
Harness
Overalls
Gloves
Helmet
Neck brace
Head restraint net
Wheels and tyres -
1 set wet, 1 set dry, various spares.
Inner tubes.
Puncture repair kit.
Wheelnuts.
Air pump.
Oddities -
Fire extinguisher
Ratchet straps
Gaffer tape
Hand cleaner
Rags
Waterproofs
Notebook and pen
Licence
Cash and cards
Spare keys, given to someone else and never left in the van.
Unless you've got a van specifically for the job, pack all this into sturdy, stacking boxes to keep it organised.
I think that's the lot.

(if I think of owt else I'll edit it more)