I can recommend starting with pitch updates (already two updates) since they decrease the risk of injuries. Then going for boosters makes sense: Medical staff and fitness center. Afterwards you can upgrade the stadium, if you are about to get promoted (one level with each promotion and you will be fine). With stadium upgrades the parking lot must be update, too (spectators want to park their cars). Restaurants come then since they bring some extra money, but not as much as stadium upgrades. Scoreboard and floodlights have lowest priority to me, only updated when I have to. Youth academy is important in the later stages, highest priority after pitch updates and boosters, but in the first levels the player quality is to bad to help your team or bring in good money, so I not build it until you are level 3/4, but then do some upgrades.