The game type will decide this too. Obviously on Slaughter you need to worry more about health boosting items than citadel upgrades, where as on Fortress the building upgrades are essential to victory. The other DGs make an important factor here too. If you know know you are up against TB getting Priests will just feed him. If you are facing Erebus the same rule applies. If your enemy is spamming a Heart of Life then angels will drive him nuts.
Now it will change with every match but if you feel confident with just a teleport scroll I recommend getting Structure I as soon as the match starts, on all games but Slaughter. You'll be able to creep up your gold pretty easily but having your structures regenerating from the beginning of the match will help deal with the little damage enemy creeps do to it over time, and will often deal with the slower start.
Basically it all depends on the game. If you are getting chased down and auto-attacked to death get boots of speed (or a wand of speed) and HP boosting items. If you are running out of mana and going back and forth to base get the Battlecrown and the Vlemish Faceguard combo. For a bit over 3k your mana problems will be mostly solved. If you find yourself not doing much damage buy some gloves to compensate. If you can't do that last bit of damage as they run away the Wand of Speed is great for a chase down to get off the last attack. When the enemy gets a Heart of Life it's practically required for you to get one as well. Regardless of what your output is the thing will heal them faster than the crystals in your base.
Overall, in the ideal situation your opponent should be the one pondering what item or upgrade purchase will help the most, as when you have the advantage of acting first, your opponent will be on the reactionary side.