Get one of the kits... like from Brown. DRDT2 <-- get this in the beginning. Makes life easy.
Then get the following additional (look for deals on these, you won't need right away)
Full set of reamers, and a .311 reamer.
hole saws. Hydraulic knockout punch is nice to have too... (will make punching holes in firewall a breeze)
compressor if you don't already have one.
measuring tools (calipers, steel ruler 1/32 or 1/64)
cutting tools. can never have enough cutting tools. vertical band saw will be used a lot. I mounted my portable band saw on a wooden custom table and have used that.