[scooby doo]

Depends on what your after, Been down this road many times now.

The td04 is a good road turbo and spools up very low but has the tendency to tail off .
Andy forest used to hybrid them making around 300-315hp and pull till redline.
The larger vf range works better on the sti from the advantage of the avcs heads which yours doesnt have .

Cheapest route run all 3 decats and 2.5in cat back , walbro , panel filter and samco y pipe , mapped 270-280hp so nice 40/50hp gain there and keeps everything from being stressed out.

Same again but hybrid td04 with a 3port and sti injectors as you will be at 100% on stocks this will make 300/300ft

Next level and most comman pretty much same as you have said ,
Vf35 is easy to get hold of will see 340hp realistically,
Sti tmic and under tray to make the most 3inch turbo back system
Will see 330-340hp but I'd expect a clutch at this point and you are now starting to strain things so if it's a higher millage car the box may decide not to like it ,

Personally I'd stick to around the 300hp mark which is some 80hp more than stock and will drive loads better , nothings strained and if you want more by a sti that's simply the better way to go, can't say it enough how much stronger the sti is out of the box ,

As others have said you will be looking at brakes , arb , wheels to accommodate calipers so the bill starts to really add up ,
I'm currently messing around with a wrx and will be taking just the 280hp and mess around with handiling