05 WRX is PCD 5x100.
18" tyre size would depend on the width of the wheel purchased, but most common 7.5 or 8" wide wheels (usually labelled as 7.5J or 8J on the wheel) will take 225/40/18 tyres.
Ride wise... on a newage I don't think there's a massive difference really. Looks way better on 18s though
Offset.... Subaru is ET51 or 53, can't remember, but I've run ET42 with no problems on a fairly standard bugeye.
Bore size? Assume you mean for the nuts? They're 12x1.25mm with a 60 degree taper i think.
"What not to do" tips..... don't buy chavvy wheels
and bear in mind clearance for brake calipers - you should be fine with WRX 4 pots but if you upgrade to Brembos some wheel designs will foul the caliper.