You shouldn't need spacers for the back, if you find an outback sedan it will be level, a wagon will have a slight ass up look, to see the difference just park next to an outback!
Just remember you need to use the 00 to 04 top hats if you use 05 to 07 struts.
No clue on impreza exhaust, I would plan on going custom. If you are trying to do this on the cheap, find a donor car unbolt the fart pipe clean it up and cut and replace each section you are looking to upgrade, muffler cats with lor restriction units ect.
The ebay 2.5rs dual port headers will fit, they take some finagling and stretching but they do fit, spend some time giving them a good coat of heat resistant paint or exhaust wrap for added insurance.
Rignt now i have parts from 6 different subarus on my car, imprezas, legacys, wrx. And a tribeca have all donated.
Next up is grabbing front brakes off an 05 leggy gt and rear 2 piston brembos off an sti. Hence the Mutant edition moniker, I was going to get one of those "5hitbox Edition" badges off ebay but the wife says this fits better