Here are the contents of my field box
note this is from memory so i am positive im forgeting something

first the box:
Black and Decker "Master Cart" Like Billy Hell's in this thread
with Hobbico power panel, sealed 12v battery, electric fuel pump.

Tools:
set of small standard and metric sockets.
*3 phillips screwdrivers (diffrent sizes ranging from small to medium
*3 standard (flat) screwdrivers (matching sizes to the phillips)
*set of metric allen wrenches (i recomend the "ball end" ones that can get be used in tight spots cause sometimes you can't get straight on for some bolts)
*set of standard allen wrenches
*set of pliers (slip joint, regular, and neddlenose)
*adjustable wrench
*Exacto knife
*Glow plug wrench
tweezers

"Field" tools:
12v electric started
*"Chicken Stick" (acually an old broken prop )
*"C" size glow igniter
power panel glow igniter
*Manual Fuel pump

Misc:
*CA Glue (thick and thin)
Screws (anything lying around ... you never know)
Thread Lock
"C" size alkaline batteries (backup for glow igniter,)
Zip Ties
Small Pieces of wood (for minor patches/adjustments)
*Props (various for diffrent planes)
Control horns, hinges, cleveses, servo horns/screw, etc
Extra Fuel line/filters etc.
extra servos, extentions, y adapters.

just about everything here has come in handy at some time at the field. I know this isnt the "primary" needs but it is something to work up to.
(I started with a cardboard box with a glow igniter, 12v batt, starter, manual pump, and a few odd tools)
**I put a * next to the stuff I feel is "needed"**

Also I would recomend buying good quality tools (snap-on, craftsman, etc) because the cheap ones are more prone to breaking or stripping screws/bolts. and stripping something like a engine bolt is a quick way to end your day

I would also recomend getting a box with wheels or a luggage cart to carry your stuff with. even if you dont have alot now evetually it will add up and get heavy..