Chassis is fine - it's a GC8/GF8, so it'll handle it.
Gearbox will probably assplode into a brazillion pieces (or at least shred a gear if you get grumpy at it). And yeah...clutch funtiems
Wiring...not the same. If you want a challenge...knock yourself out
Brakes...for the love of all things good and holy, CHANGE THEM.
Suspension...you will want some upgrades, but that's a duh if you're getting it ready for track.
Ultimately I'm NOT a proponent of modifying the sh!t out of an HX, as that's exactly what I'm doing and it's the hard road and then some. But if you intend it to be more than 'just' an STi - ie a track car, caged and everything - you've got a perfect starter shell. You'll ultimately change out anything that's HX, WRX, STi for track-worthy aftermarket stuff anyway, so better save the money and go for the chassis code, not the final trim.
But for the love of all things good and holy, don't try and make an STi out of an HX20s ;D