If you plan on tracking your zed, my list would be
1. tyres
2. brakes
2b. both 1+2 together would be great!
3. suspension
4. seats / harness
5. engine cam, induction etc
6. LSD diff
Suspension is third because I assume you have a good setup to begin with not shocks that are worn / rooted or soft saggy original springs. If so, replace before tyres!
Further engine mods is when you have reached the limit of your setup; track times are consistent..