1-Screw the coilovers. Get AGX or GR2 struts if yours are shot.
2-Look for (and get) front & rear strut tower braces.
3-Get decent tires, a "+1" to 16" rims can be decent & not too expensive if you already need tires.
4-Skip "engine mods" for a 2.2L unless you want to get into either head porting or a 2.5L SB swap
5-Roof rack, either OEM Subaru or look to Thule.
How do I "know" these things? My wife is an original owner of a 1999 Impreza L wagon.
What part of the country are you in?? It helps to "fill in" your location in your profile.