Hi Hopefully I can be of some help as have had a bit of a wheel fettich over the years (on my 6th set).
17 inch rims :
7.5 x 17, PCD 5x100 and offset ET53 is the official size from Subaru
You can fit ET48 which will bring the wheelout a little flusher with the wheelarch. It looks better but Prodrive did alot of research and the ET53 was deemed the best handling.
Tyre size needs to be 215/40x17 or 215/45 not the 205/40 that alot of wheel/trye packages come in. 215/45 again is the preferred Prodrive setup.
18 inch rims:
7.5x18, PCD 5x100 and offset ET53. Alot of aftermarket are ET48 and you can get away with this on 215 wide tyres.
8x18, PCD 5x100 and offset ET51. This is as wide as you can go as the 8 inch rim needs 225/35 tyres. 215's on the 8 inch rim are too stretched and look a but euro/retro. Any wider then 225/35 and you foul the arch.
Be wary of 2nd hand newage wheels/tyres for sale as the tyre profile is taller then that required for a classic. Also the 2005> are PCD 5 x 114 to accomodate larger wheel bearings.
Have you had a look at
http://www.rarerims.co.uk/
Some nice wheels,good prices and some are really light.