I always liked using Soldarin's ECL system (
http://soldarin.tripod.com/id5.html) when balancing racial HD with classes, because the core 1HD=1ECL +LA system just never worked. According to his system, a Dragon or Outsider HD is worth ECL+0.8 (for the HP, BAB, Saves, and skill points, I mean). You're not getting any class abilities/spellcasting, so they're still a little weaker than a class level. Of course, this system was made back in 3E, when monsters got fewer Feats, so you COULD argue it's gone up.
Doing the math: every Deva HD you replace with a Druid level gains you 0.2 ECL, and that's assuming you don't get ANY of the angel abilities. The 12 HD a normal Deva has, then, would be worth ECL+9.6 (call it 10), meaning all those other abilities are actually worth ECL +10.4. You could try breaking it down further if you didn't want every ability, but I'd say then that the full package would require 10 levels, and that's assuming that for those 10 levels you didn't give ANY hit points, BAB, saves, or skill points.
Bottom line: I don't think it's possible to let a player become a true Deva without becoming horrendously overpowered. It's not a question of whether the player is a powergamer; it's just inherently too strong.