hub adaptor - required to adapt the splined steering column to the momo bolt pattern (a generic pattern typical on omp, sparco, etc., steering wheels).
quick release (QR) - this mounts between the steering wheel and the hub adaptor (ie, momo bolt pattern on each half). street car? then you will need one that allows you to continue to use your horn (NRG make one).
steering wheel - tends to be generic momo-style bolt pattern, unless you go with an atiwe or something with the three bolt pattern.
if momo then presume no airbag. in that case you need a resistor to fool the computer, or run with a warning light (if I recall correctly you need a 4.7 ohm half watt).
make sure the hub you get is designed to work with the 986/996 - the steering column is a little longer on these cars and you often needa spacer to make Porsche hubs fit.
check out fvd for Porsche-specific packages ...
FVD Brombacher Online Shop - FVD34710003 - Steering Wheel "Rally Style" black suede, incl. retro fit adapter and horn pad w/logo - Tuning → Interior → Steering_Wheel → Sport_Steering_Wheels