Your biggest feat will be pulling both front wiring harnesses from both cars and getting them merged together. 06-07 Impreza (base,wrx,sti) are CANBUS so a 04-05 swap is easier.
https://www.ebay.com/itm/Whiteline-Front-Standard-Control-Arm-Bushings-for-Subaru-Impreza-Legacy-Outback/273607072650?fits=Year%3A2001%7CModel%3ALegacy%7CSubmodel%3AGT+Limited%7CEngine+-+Liter_Display%3A2.5L%7CMake%3ASubaru&epid=253491083&hash=item3fb4409b8a:g:M2gAAOSwZupcENYa
looks like price has gone down
You have remove the intake manifold as well to take the heads. Oil pan is easy to remove. You'd also have to remove the cams to take the heads off as well. If you plan on reusing every make sure you put every thing back in order. IE the cam caps and stuff like that.
03-05 foresters NA and impreza NA headers are the same but the rest is not. Bosal makes Carb Compliant and makes exhausts for subaru so it will fit well. Will probably out last any ebay cats. They do cost around 400$ though. you get what you pay for.
lucky you don't get rust. Those ball joints are a PIA around here. That bushing in the back is hydraulic so best get a arm with one on it or buy the whiteline kit for 120$ changes it to poly. You can also buy the energysuspesnion bushing kit for 85$ if you find a good used arm. Rockauto has them for a decent price.