Is your impreza JDM? If not, then the swap should be fairly straight forward because all the harness work should already be done. Some things to consider would be
1) If the foz is auto, swapping a manual transmissio into it is a bit more complicated, so it may be easier to looks for a manual foz. I know this because I have an auto forester and have looked into putting a 6spd into it.
2) are you planning on getting an XT? The 04+ can command a bit of a premium. If you get the XS, it will have most of the comforts of the XT (leather interior, giant moon roof, heated seats etc...) and will be significantly cheaper. I don't know for sure, but I think you may have to find a turbo crossmember, and the rest should be easy.
3) I'm with LDS. I'd consider swapping an ej257 into the car instead. Sell your ej207 swapped impreza and use that money towards the foz. This will probably depend on which ej207 you have. Is it a version 7+ (does it have AVCS?). If it's a v5/6 I don't think it's worth it.
4) you can check out my build thread. I just rebuilt an ej257 and dropped it into my forester. It may help... Although it is pretty engine heavy at the moment.
Hope there is some helpful info in there