it all depends on your target budget (setting up/running), timescale, peak bhp/ftlb, application (road or track), level of reliability and effort ur willing to put in.
the higher you stress the whole cars 'package', the closer parts reach their operating limits so will require investments in upgrading/refining various supporting systems to prevent an expensive catastrophic failure.
to reach say 160bhp (bout the safest reliable max a stock cast piston would run under track stress imo) with a t2/t25 hybrid turbo like mine for street & track use as an example, engine-wise I had to upgrade:
fuel
98ron super
walbro pump
370cc injector & rail & regulator from 200sx
air
pod filter
catch can
custom stainless welded inlet plumbing with insulation wrap
sparco full width intercooler
saab 9000 recirc valve
GTIR MAF
bypassed TB coolant
custom teflon inlet manifold spacer gasket
electronic
Nistune daughterboard tuned on dyno
wideband sensor
boost gauge
cooler bkr6e spark plugs
exhaust
custom manifold
custom 2.5" exhaust system & straight through backbox
oil
custom stainless braided oil feed line from engine head oil port
custom stainless braided oil drain line down into sump
cooling
thicker 26mm radiator from automatic k11
power train
200mm helix organic clutch & PP on a re-drilled flywheel