the sti spoiler will fit straight on, but there are more fixings you can use if you don't mind drilling the boot. i fitted my sti spoiler a year ago just using the wrx fixing points and not had a problem. you will need the stronger boot torsion bars too, otherwise it will be heavy to open the boot :)
a prodrive back box won't be much louder, have a look for a cat back system. 2.5" is the std size, it should unleash some of the growl if you get a deresonated centre section, or go for a 3" if you want it louder
be careful with the dump valve, some can cause running issues ;)