think 3 basic catagories;
1) hand tools
2) meters and testers
3) power tools
1) get a few catalogs, or even ask a supply house for thier best sellers, sometimes you'll see a fully stocked tool belt.
Get a comfortable tool belt, as you will most likley assume the position of a pack mule in heat after 6-8 hrs a day should you not.
2) a volt-tic, VOM, amprobe, megger, impedance tester, circuit tracer, not necessarilly in that order
3)big drill-right angle , sawzall, unibits, hole punch kits,just to start off......this is the biggest list & never really ends