What stats for leveling an engineer?
If you equikittent you unequip your weapon, anything else stays the same. You still get the stats bonuses from armor, amulets, rings, earrings etc.
Anything that deals direct damage (which is everything but conditions) benefits from Power according to the following formula:
(Current Power + X Power) / Current Power = Damage gain
So, if you double your current amount of Power your direct damage will be doubled.
Condition Damage increases the damage of each condition differently, full list:
http://wiki.guildwars2.com/wiki/Condition_Damage
For leveling until around 40-60 going primarily Power and/or Condition Damage and secondarily Vitality and/or Toughness is a solid route to go.
Bomb Kit is properly the best until level 60, then Grenade Kit surpasses it.