Well, the Prodrive springs were designed to work with the OEM WRX strut damping rates, so they should match your struts *fairly* well. Yes, I do believe the rear springs are not compatible between MY02-03 and MY04, sorry.
What size tires are on your brother's 17"s? It should be printed right on the sidewall of the tire.
FWIW, I have Eibach Pro Kit springs on my wagon and love them. Another "bang for the buck" initial mod is to find someone with a used stock WRX sedan rear sway bar and install that.
Welcome to the club and good luck.