Sally The Subaru

Hi, I have a 2008 base model Impreza with a EL15 and manual FWD. I have found an EJ20 manual box all axles and rear diff from an Impreza S-GT. Will this bolt onto my chassis with the current sub frame? Engine wise I wanted to get an EJ257 but all the ones available are NZD$4000. I also looked at the much cheaper EJ253 but that has a significantly lower power output and is non-turbo. I also am considering a turbo EJ20 to emulate the S-GT, but ultimately want the built to be on-par with the STI. Also have had trouble finding an ECU for these engines except the EJ20 ECU which would come with the gearbox. Is there anything I have over looked since I basically know spoob all or if anyone has done something similar is there are other parts I need for this or a list of parts you used that would be greatly appreciated. Thanks, Harrison.