Welcome to LegacyCentral, Muiretta!
The '02-'04 16" wrx wheels offer a noticeable imrovement in handling. They are the easiest ones to find used. Tire size will be 205/55 16 or 205/50 16. With tires, many times they sell used for $200-$300 for the complete set.
KYB's GR-2 struts work well. You can find them on sale new on ebay for about $250-$270.
The GR-2s will raise the front end of the car so some owners choose to install lowering springs on all four corners, such as by Whiteline, H&R, etc. New, they cost about $350-$400 but if you don't mind used, they will be more affordable.
If you want improved braking, used front '02-'05 wrx brakes are a relatively inexpensive way to go. $150-$250 for rotors, calipers, brackets, and pads. Brake fluid, stainless steel brake lines are extra cost. You will have to use the 16" wheels to fit over the wrx brakes.
Thicker front and rear sway bars will improve handling. For your NA Legacy, the second generation Legacies had a thicker front sway bar that will fit and turbo Legacies have a thicker rear bar. Probably $20-$80 used each. Or, Whiteline makes them in different thicknesses but they will run $275 or more each.
From time to time you can find these parts for sale here on LegacyCentral, your local Craigslist, NASIOC, and other Subaru forums, ebay, etc. For new Whiteline products, you can check ebay, PDM Racing and other places.
This list is by no means exhaustive. But it will get you started. To find out more about upgrades and where to find them, you can use the SEARCH feature to call up threads and posts by Legacy Central members.
If you're serious about getting a sport sedan and your '91 wagon doesn't really need the upgraded parts right now, it may be worth your while to get the ss first and start upgrading it rather than spending your funds on the wagon. The greater torque and hp of the turbo engine makes driving a Legacy much more enjoyable! You may end up like many of us who keep upgrading because it makes driving much more pleasurable. That means it will cost you more money so rather than spending that on your wagon, you may want to put it into the ss.
Have fun modding your car(s)!
