In stock form the WRX is a bit pudgy and understeer prone. What kind of track are you running at? The AWD sure is handy in off-camber stuff and any crummy track conditions (rough pavement, wet, etc.), don't let anybody tell you AWD serves no purpose. Sure, on a flat, dry course the extra benefit of AWD is hardly worth the weight, but on a course with less than ideal conditions the AWD makes it a blast because you're still just as fast and can typically run a pretty flexible line compared to the FWD/RWD guys. While billy3esq might have had fun in the wet with his Miata because he could finally do a powerslide, a WRX driver would be having fun because he could power down coming out of the corners and go forward instead of sideways.
Standard track prep (assuming you're not running crazy speeds) would start with a healthy dose of camber and at least a rear bar or maybe a pair of bars depending on your budget and preferences. When you get into struts, the sky is the limit, so it really depends on your budget, what you want to get out of it, and if you're just driving for fun or if you're competing. Oh yeah, and of course good tires would be necessary and you may want to stick some fatter meat on there with aftermarket wheels, but that should go without saying. Some guys that track their Subies like to get camber plates so they can quickly adjust the camber for track days and not destroy their tires during typical commuting, so if you're using it as a daily driver that'd be worth considering. They really start to shine when you shed some weight, so depending on what you want to use the car for you might consider stripping it out a bit. It's a lot better to remove weight than to add boost when you want to start going faster IMO.
In terms of reliability, leave the powertrain stock and you won't be disappointed at all, if you start to modify it you'll need to keep a few things in mind to keep the reliability up. If you crank up the power (tune, exhaust w/ up and downpipe, etc.) you'll want to be nice to it on launch and 1-2 shifts, in particular, but avoid powershifting in general. I don't really think hard shifts make much sense in any case for just having fun, but some guys get a rise out of abusing their trans. The OEM clutch is a bit soft in these, so really hard launches with a bunch of power will take it out in short order, and as mentioned the 5 speed doesn't like hard abuse with added power, particularly 2nd gear (where most guys at the drags break it trying to get a super fast, super hard shift into 2nd).
You may want to go into more detail on what kind of budget, prep level, track days/competing, etc. that you had in mind to get more specific info.
Bryce