In general, the kit has everything except any drivetrain-related pieces (except axles and CVs), tires and battery.
So you'll need to source the engine, engine controller and harness, transaxle, clutch, starter, fuel system (including fuel pump(s), swirl pot, etc.), and other small misc parts related to the drivetrain.
RCR can supply adapter plates for popular engine/transaxle combos, as well as some specific flywheels.
You should plan for the usual stuff in a build like this, like a reasonable set of tools, tape, fluids, occasional fasteners, small brackets, and so forth.
Aside from the obvious options (you will need a lift kit for the street, for example), there really shouldn't be too much more, unless you start to deviate from the standard build, as so many of us do to our eternal regret.
Just kidding about the non-standard builds- that really is what makes each car different, and special to the owner/builder.

But it's important to understand that going off the beaten path always takes more time and money.