look for 15" wheels, so you can buy castoff gravel tires cheap from rally teams. shop around carefully, you can find 15s that fit over 02-03 WRX brakes ok. you can make many other 15s work with wheel shims + a little grinding of the calipers if you're determined to go down that route. realize that gravel tires have very specific tread compunds and are NOT usable for normal street driving and cold weather use. but in the dirt there's nothing like em.
look for 15" high-brake clearance rims from brands like Team Dynmaics, Revolution, Speedline, Compomotive or Enkei. sometimes rally teams sell used rims, but they'll be pretty beat up.
16" gravel tires are super expensive, enough to offset the higher price of the 15" gravel rims (which again let you use cheaper 2nd-hand gravel tires)
