You need to do some more research.
The STi TMIC is not a good option if you want to go with a fairly large turbo. It's good for an STi sized turbo (VF39) but it's still fairly limited. There are larger aftermarket intercoolers out there from Perrin, APS, and others. You don't have to get a front mount to get something bigger than the STi intercooler.
The VF22 is not that great a turbo. It was popular early on because it was one of the few turbos that used the WRX's layout. Now, there are a plethora of turbos out there. I would recommend an E3-16G turbo. It will cost less, spool much faster, and get similar peak numbers.
18 PSI is just a number. Power is proportional to airflow, not boost pressure. I suggest you find a good tuner who can work with whatever setup you get and come up with a good, safe, and powerful tune. I would not have any specific boost pressures in mind for what that means.
Your injectors are very small and barely enough for the stock turbo. If you go with a substantially larger turbo, you must change the injectors as well as the fuel pump so that you have enough fuel capability to actually increase power. 650cc is appropriate for an E3-16G. Different turbos may require more or less.
The EJ205 handles additional boost and power very well. It's quite happy in the 300whp range. I wouldn't worry too much about internals unless you start messing around with more aggressive mods than a bolt on turbo swap. The part of your car that's most likely to break from these mods is the transmission, not the engine. The stock 5-speed is pretty weak and many people have problems at stock power levels let alone something twice as high.
04 STi 16G powered lagless fun
04 WRX: SOLD