I believe we have the same 2.2l MAP motor.
Here is a link with another link to a few simple & cheap mods I did when my OBS was N/A that improved acceleration.
I like my little L wagon, but...
Handling can be improved with springs, struts, Swaybars & endlinks.
I had AGX struts / Eibach springs on my OBS that lowered it a bit & allowed me to dial in more neutral handling, but the best bang for the buck was the addition of a thicker rear sway-bar (mine 19mm RSB came from a '05 STi (I read it would not fit after I had it on my car - go figure...) & cost about $50).
WRX wagons have a RSB that is thicker than the 13mm rsb on your OBS right now & these can be found for about $50 complete.
The simplest upgrade for the braking system is the use of decent fluid (I use the Valvoline dot4 synthetic stuff) + good brake pads (I use hawks).
A cheap upgrade for the braking system was the use on Legacy 2-pot front calipers & rotors. I got a set of calipers & brackets from my local u-pull-it on the cheap, the system bolts in & will net you bigger rotors & better calipers that still fit under stock 15" rims.
This should be a OK start for you, on which you can build/improve.