coming from an "old" and been around for a long while subaru fanatic...
The build for the 08 impreza will cost 2 metric shit tons...to get into it however will be most of your big expenses. Get a full blown EM, rip your engine out to replace with a $8-10K longblock, also why the engines out, swap to the turbo model front crossmember, then buy a UR 42R turbo kit, then comes about $1.5k in fuel system (not kidding)...I haven't gotten to the drivetrain yet.
You will need to buy an R180 rear end from an STi, have custom DSS axles made to fit the new platform as you have different hubs (or swap completely to the New STi hubs, lat links, brakes, etc and still buy DSS axles), clutch (I'd recommend a twin or triple plate if going at it right), source a pull type trans from an 02-05 wrx and have a PPG gearset installed (or get an sti 6-spd and hope you don't blow it up)...I'm assuming you are chasing a 9 second bike with this list though...

If you just have a 600 to beat, get an 08 STi, slap on injectors, fuel pump, FMIC, TBE and GT850R...you'll be into the 10's that way