The most important to upgrade along the way is your weapon by far. Constantly equip weapon upgrades and consider tossing a cheap Tempering on them to increase your damage.
It's unlikely to come across many of these Legendary Aspects while Leveling up. Luckily you don't need any of them to level with ease. Use whatever you have that helps in anyway and look out for the ones listed in the planner. Always put the strongest Aspect you find on your Amulet or Weapon for the boost to it's power.
Most of your items along the way aren't going to have 4 stats on them and It's usually more important to equip upgrades as opposed to chasing the suggested bonuses. To put it simply you want to be looking out for:
Offensive
- Dexterity
- Damage (or Vulnerable Damage)
- Critical Hit Chance
- Critical Hit Damage
- Attack Speed
Defensive
- Life
- Life on Hit
- Life on Kill
- Thorns
- Armor
- Resistances
- Movement Speed
Tempering
In general you can Temper decent items for cheap early on and can put Damage, Thorns or Toughness options. Keep in mind It's common to replace items often while leveling up, so it may be wise to hold off on Tempering all together until you're around level 50+ to save time spent in town as well as materials.