You definetly need to upgrade your fuel pump. Walbro is the pump I am using (at 4 psi) and its a high pressure unit. This will supply PLENTY of fuel for the extra air the turbo is creating.
I am also using a Stromung axle-back muffler. I had this before my setup went on and loved the sound and didn't want to change it. So, I bought a WRX downpipe, midpipe, and rear section (no muffler) for 100 bucks and it works great. This will fit onto the RS, though the flanges for the Stromung and the WRX rear section will have to be mated, and your front O2 sensor will need to be placed BEFORE the cat in the downpipe. Your rear O2 will screw into the WRX rear bung perfectly. It works so well I didn't get a check engine light. The front one will have to be welded in.
I am running the internal wastegate on my Legacy turbo right now and it works great. Just remember, the more you can do before you put the turbo on, the better. If you can, spend the 300 on the Tial external and have it fabricated onto your uppipe and downpipes. The WRX turbo your ordered right now is going to spill its guts at 8 psi, much more than you want. If you use and external unit, you will need to have the wastegate gate on the turbo welded shut, to make it inactive, on top of removing the whole wastegate assembely. Most MBC's have proven unrealiable, due to the fact that theey tend to spike and blow engines. There is one that seems to be safe, Subachad uses one on his 1.8-T. Its called a Dawes Device boost controller and its like 35 bucks. This will overide the internal wastegate.
Boost controllers and wategates do the same job, just differently. The four options you have is a MBC, EBC, internal, or external. I personally trust the external wastegate over all, but thats just me. Definetly buy the TIAL unit, its the best.
As far as getting a lower pressure spring for the internal wastegate, please tell where you found one if you decide on one. I think thats a good, cheap option, and something I would do myself if I knew where to buy one. I haven't really looked though, and I'm not sure if one from a Garret turbo can be used on IHI turbo.
Graham