I had four 5.0 Fox body's back when they were new (82/83/85/93). If memory serves me correct, they liked:
- intake manifold
- shorty headers
- gears
I think those should be your first buys.
Of course, seeing your car is an 89 and is 23 years old your other option is to keep it/return it to 100% stock. I love seeing older bone stock Fox Mustangs!