on the 1jz i dunno but the rest i can help
the downpipe. especially a larger one, lets the turbo spool faster with less resrtiction, and lets the exhaust and the waistgate air mix more easily and flow faster, yes i think it is nessassary for making big power.
BOV, i like my hks ssqv type bov, its adjustable for hard "PSHHHTTT" or soft "pssssssssssstt" but it does wistle a bit if i let of easy on the gas. Its so loud ive made people jump off the sidewalk in fear for thier lives. hehehhe i love it...
The Intercooler i went on ebay and found a dude selling polished spearco cores with treadstone end tanks for about $450 very nice IC's and i made my own intercooler piping from raw mandrel tube from
http://www.kteller.com. the best price i found,
and a big K&N air filter .
from there it depends on what kind of power your planning to make? Metal HG, fuel injectors, fuel management sys. larger throttle body. boost controll , search for bpu's. basic performance upgrades