[GeeTwo]

The lists above are quite good, except that they forgot screwdrivers and measuring devices as #1 priorities.

Starting from scratch, my list of must have tools would include:

Top Priority:
Tape measure
Square (at least a "speed square", preferably also something much larger)
Straightedge (which might just be a length of aluminum angle)
Pencil (though ultra-fine sharpies are even better)
Hand Drill, with basic set of twist bits plus a step bit and perhaps select spade bits (which cut aluminum nicely). Screwdriver and allen driver bits are also a great addition.
Hacksaw
Wood Saw (preferably a back saw with a miter box that can cut 1"x6" board and 2-1/2" round pool noodles)
Screwdriver set
Hex/Allen driver set (those "swiss army knife" sets are wonderful)
combination wrench set
Basic set of files, including flat and round
Knife
Scissors, good enough to cut heavy bumper fabric among other things
Pliers: slip joint, needle nose, and diagonal cutters at a minimum.
Automotive terminal crimper
Basic wire stripper
Staple gun (I just can't imagine making bumpers without staples)

Next-level priorities would include:
Ratchet wrench set
Electric Saw (either miter chop saw or portable band saw; both is even better)
Electric "hammer drill"
Drill press (benchtop is OK, floor model as an upgrade)
Calipers (vernier if you are strapped for cash; digital if you can afford it)
Dremel tool kit (rarely the best solution, but useful for lots of stuff!)
Blind rivet setter
Angle and/or Bench grinder (preferably both)
Bolt cutters (useful for rough cuts in metal stock as well as bolts)
Anderson Power Pole crimper tool
Dupont (0.1" pitch header connector) crimper tool
"automatic" wire strippers (which grab, cut, and pull on a single squeeze)