Is loredrake allowed? Two free levels of casting sounds good.
My issue with Eldrich Master is that with the way sorcerer casting works, adding a spell list wouldn't also add it to spells known.
Rapidstrike and Improved Rapidstrike tend to be good on dragons.
Eberron archtypes are very nice, but I am not sure about Loredrake. As it stands, I need 4 Sorcerer levels to pair with the first 6 racial dragon HD on the steel dragon. Loredrake would drop that to 2, which would allow for two other base class levels (Battle Dancer and Marshal maybe). Child of Eberron might be better on the Steel dragon to add the druid spell list access or any of the cleric spell casting list archtypes to the Tome Dragon. I might be more partial to a Forgotten Realms setting though if the free metamagic trick from casting in a Church of Mystra works (with an Acorn of Far Travel spell of course). That would eliminate the need for divine metamagic.
Eldrich master both grants spell access and adds some spells known from levels 1st through 5th. 4 levels on the Steel dragon would add 5 1st, 4 2nd, 4 3rd, and 2 4th level bonus spells known. Not as many as the Sand Shaper recommendation, but you get to pick which bonus spells you get. Not to mention that Eldrich master boosts your effective caster level for spell level based effects which is not the same a caster progression so stacks in gestalt. Also I could go with wizard spell casting from the spell hoarding dragon magazine psychosis, which would mean I could get all the spells I can buy.
mercury dragon...
sand shaper would add spells known, could even play as a dragon with sand affinity for other interesting synergies.
Not sure about the Mercury Dragon recommendation. It has far less spell casting levels and does not add anything over the steel dragon except different domains.
dracolich is only la+4 (draconomicon), some niiice goodies in there 
Dracolich is very nice, but the other player is toying with this template or the Walker in the Waste PrC.
I guess there are in the running:
Steel Dragon with Lore Drake Archetype
Cleric 1/Sorc 2/Battle Dancer 1/ Marshal 1/ Eldrich Master 4/ Heirophant 5/6 Spell Casting PrC (Not in this order).
Escalation Mage maybe fitting following the Shadow for Lore Drake, but not too impressed by it.
Tome Dragon with Cleric Sovereign Archetype
Cleric 1/Dread Necromancer 1/Sorc 1/Battle Dancer 1/ Marshal 1/ Eldrich Master 4/ Heirophant 5/6 Spell Casting PrC (Not in this order).
