1):The obvious tools.
a): Philips (crosshead) screwdrivers, Not just one, its worth buying a few sizes to save stripping the screw heads, Using a too small driver will ruin the screw.
b): Pliers, long nose, stubby, and side cutters all have different uses, so dont try to use the wrong type on your car. It usually makes a mess of whatever you are gripping.
c): Modeling knife(s): There are many use for the old trusty modeling knife. Building especially, body trimming, and many trackside uses that you will make up as you go along!
d): Allen drivers/keys: Make sure you know wether your car has metric or imperial allen screws/bolts. As with screwdrivers it is very important to use the right size tool for the job. A badly stripped allen screw will have to be drilled out, or cut a groove in the head with a dremel to use a flat head screwdriver.
It really is worth investing in a decent set of hardened allen drivers, as the cheap allen keys allways seem to round off.
e):Compass cutter: Only a build tool this one but it make your life so much easier when trying to cut out wheels arches. Polycarbonate bodys split and tear very easily, if there is any sharp angles, instead of nice smooth round arches.The cutter makes the body last a lot longer by making it stronger. (If you use it properly).