If they still make the cargo tray, get that. Instead of the subaru short shifter, I would get the Kartboy model. I would get the security system (thats the shock sensor right?) and the solid splash guards. The battery warmer and engine block heater aren't really needed. My 04XT still starts just fine on the oem battery in 2- degree F weather, and it's parked outside (the heaters don't make any sense to me though because there are no outlets in my parking lot!) There are many more easier to mount, cheaper, and better boost guages on the market, so I wouldn't recomment the subaru one. And the rear diff is pretty much the only thing you WON'T hit if offroading (if you're going to really be offroading at all I would buy a primitive skidplate (they're a vendor on this board). However, when buying the car you could probably get them to throw one or two things in free.
04xt/4eat, 16g, STI bits and bobs