Model you're looking at is called a 'peanut' (or 'blobeye' according to some US sites).

'05 has upgraded hubs and runs 5x114.5 lug spacing, '03-'04 runs 5x100. Important for wheel choice, not something you'd buy a car specifically for. '05 also has a bit of a rear quarter panel flare I think, just for styling differences. As previously mentioned, '05 is the last 2L STI (that we got anyway) and has pretty much all the fruit, so is probably your best bet.

DCCD is Drivers Controlled Centre Differential, read up here: Subaru Drive Performance Magazine : DCCD - Driver Controlled Center Differential

No idea about prices and kms, but I'm sure there are plenty of people here who can provide some insight!