a friend of mine has a cavi with 3 turbo chargers linked up in it. fast as hell. $2,000. it'd be hard to find an sti for under 18,000. I'd just buy a caddy for like...$6,000 and put a system in it, buy 24's, and new paint job.
Unless youre a fan of european cars. They have great engineering but theyre expensive as fuck to find parts for.