If you keep to a 2.5" system then you can swap the centre cat in and it should pass. 2.5" is good for about 350bhp, and to be honest anymore than that and you will be budgeting for a new gearbox and possibly a 2.5 block after your one explodes.
Also take into account the logistics - when you decat the up pipe you may as well do the headers to cut down on labour and if you want them ported they will need to be sent off, so the car is off the road for about a week. Then when you have done all you want to do it will need mapping sharpish so you need to know when the mapper is able to do it and work around this and when you plan to get all the work done.
I have been looking for a vf34 for a while for mine and have just about given up, so now I have decided to get a turbo off Andy F, they are quality items and reasonable BUT for most turbos he does you will need to send yours back in exchange. He does do new ones but you will pay more for it and if you want the turbo before sending yours off a £200 deposit is payable.
Uprated fuel pumps are easy to get, about £80 new, so no problems there, and sti injectors come up for sale when sti owners want to upgrade, just keep an eye out. You can get them new, but obviously more expensive.
You wont need a front mount intercooler, an sti one if perfectly adequate, but if you get one secondhand bear in mind you will want the undertray and you will also need to replace the intercooler y pipe as yours will probably burst when its being mapped.
Good luck mate, you are doing what i plan to do, the financial side of it was looking reasonable untill i had to pay for a holiday to las vegas. And remember the money side is only part of it, a lot of time will need to be spent too. And if you manage to get a vf34 be prepared for me hunting you down.......