it all depends on how much boost you intend to run, how u use it and how reliable/safe u want it.
for very low & briefly used boost, the absolute bare essential is:
custom manifold
turbo with +ve pressure actuated WG
oil feed (theres a feed from the front of the engine head beside the manifold)
oil drain to sump
custom downpipe
preferably air filter
intake piping from compressor to TB
next essential when possible is invest in a wideband to monitor the AFR are safe (14.7afr from idle to cruise load, below 12afr at full load)
more boost pressure = hotter air inlet temp = closer to detonation
on WOT boost for long periods = hotter air inlet temp = closer to detonation
to reduce detonation you can increase fuel octane, reduce compression ratio, reduce intake temps with a cooler or only use boost very briefly to prevent heatup
there's 2 strategies of cooling btw. one is to "heatsink" where the air/air cooler rappidly transfers the inlet heat out to ambient airflow fast as possible (ideal if ur on boost constantly like on track). other way is "heatsoak" where either a densely walled cooler or a air/water cooler soaks up the inlet heat to a point and then gradually dispurses it to ambient air during off boost period (such as daily driving or drag strips).
as boost increases, the airflow & fueling begins to run beyond the stock ecu mapping and afr becomes dangerous so alterations to the sensor readings & fueling are required (either ecu daughterboards, dedicated ecu or franks methods
)
further increasing boost begins to max out the MAF, injectors, fuel regulator, fuel pump so they would need upgrading
dump/recirc valves are preferable at high boost to prevent boost surge from damaging the turbo shaft and turbo seals and aids boost response during upshift
some turbo's have waterjackets which helps with heatsoak when turning the engine off. its not an essential, just means after heavy use u should cruise few mins off-boost to cool down the system before turning off (which is what happens in normal driving anyway).
note this is only advice from my knowledge and is not an absolute guide