You want a wyrmling Steel Dragon, not an Old one.
It's only wyrmlings that have the LA of +2. Older versions have more level adjustment. In fact, I think the oldest age category that has a listed level adjustment is Young Adult.
Furthermore, those 25 HD you mentioned for an Old dragon? Yeah, those all have to be racial hit dice.
Also: Unearthed Arcana Dragon? What?
My bad, Arcana Evolved (formerly known as Arcana Unearthed). Their dragons are more customizable. As for racial HD, that's the point. I can always Epic Spell in any Ex or Su I want, or an Ex or Su that emulates a spiffy class feature (looking at you, Swiftblade and Abjurant Champion), so the idea is giving all of that the sturdiest skeleton possible. And for raw power, few things top a dragon, especially in regards to never, ever being without that power. Even totally denied all it's magic and Su abilities (dead magic world), the damn thing can still fly and rip things to shreds
Granted, a Wizard or CoM can probably do all of this anyhow via shapechanging, but I wanna see what we can do with a real
nonkobold dragon. The Fucking Uberdragon (full title
EDIT : Also, ultimately the ideal dragon is one an archmage makes. Once I find my Transform seed base, I'll post up what my Iconic turned himself into in a high epic game. If anyone wants to play Epic Spellcaster, feel free.